Highlighted Enquiries

The Climate Science Rapid Response Team has received many inquiries. A few examples are highlighted below.

3 September 2013


We are working on a package of stories to coincide with the 10-year anniversary of Hurricane Isabel on Sept. 18. I will be doing a story on the question of whether hurricanes have become more numerous in recent years, and whether we should expect more (or more powerful) hurricanes in the future. I have a few days to work on this story, around other duties. Here are my main questions:

1. Are we having more Atlantic hurricanes than just a few years or decades ago? I seem to recall some people saying we are now in an active period, which may or may not be related to climate change. What can you tell me?

2. What does the research tell us to expect in coming decades? I gather that there is some thinking that hurricanes may become more powerful because of climate change but not necessarily more numerous. Is that correct? Can you elaborate on why this is?

If you want to respond by email, that\'s fine. Or if you just want to pick up the phone and call, that\'s fine, too. If you respond by email, I will probably call eventually to fine-tune some points.


3 Responses:


There is no question that in the North Atlantic region, the past twenty years have been very active compared to the previous twenty years. Hurricanes have been both more frequent and more powerful. Ocean temperatures in the summertime tropical Atlantic have also been warmer during the past twenty years.

It used be though that this was part of a natural cycle of ocean temperature. But mounting evidence suggests that the main culprit in this case in manmade sulfate aerosols, which, especially during the 70's and 80's, cooled off the tropical Atlantic and reduced hurricane activity. Largely as a result of the Clean Air Act, the sulfates have diminished over the North Atlantic, allowing it to warm up. It is also possible that increased greenhouse gas content contributed to this warming and heightened hurricane activity.

There are two points on which most of the community of scientists working on hurricanes and climate agree: 1. The incidence of the most powerful storms (Cat 3 and higher) should increase, particularly in the Northern Hemisphere. (This is NOT to say that they will increase everywhere.) 2. Hurricane-produced rainfall should increase noticeably. These two factors, together with storm surges produced by hurricanes, account for most of the destruction and loss of life in hurricanes. There is much less agreement on a point of considerably less concern to society: the incidence of weak hurricanes. Some models and theory predict that this will actually drop, while a minority of studies (including my own) suggest that these may increase along with the stronger events.

Hurricanes are fueled by the difference between the ocean temperature and the temperature of the whole troposphere. This difference is maintained by greenhouse gases, and so when their concentration increases, so to does that difference. Thus hurricanes can become more powerful. At the same time, hurricane formation is inhibited by dry air in the middle troposphere, and global warming generally increases the dryness of this layer, hereby inhibiting hurricane formation. This is why most models predict a decline in the overall frequency of storms.


I think the literature has been fairly conclusive in recent years that hurricanes of the future may be more intense in a warming climate system but not necessarily more frequent. The discussion starting on page 59 of this document (draft US National Climate assessment, http://ncadac.globalchange.gov/download/NCAJan11-2013-publicreviewdraft-chap2-climate.pdf) discusses what we know very clearly but please note that it may change some (not much) in the final version due this fall.

Yes, we are in a natural active phase of the Atlantic Multidecadal Oscillation but that does not mean that it explains all of the variance in the system. I think the latest literature takes the AMO and human contributions into consideration.

There is a very recent paper by Dr. Kerry Emmanuel at MIT that suggests and increase in intensity AND frequency (but there are caveats). This bucks the trend that intensity would be the primary response would be intensity change. See here: http://www.motherjones.com/blue-marble/2013/07/hurricanes-global-warming-kerry-emanuel

One thing that I add to the discussion is that we have to be aware of the fact that it won't take really strong storms to create a hazard and loss of life in coastal regimes in the future. Superstorm Sandy at landfall was NOT a category 4-5 storm, but it had about 10-12 inches more sea level to inundate the coast with because of rising sea levels. This underscores, in my view, that weaker storms can be more of a problem for that reason alone.


I'll have a go at answering your questions:

1. Are we having more Atlantic hurricanes than just a few years or decades ago? I seem to recall some people saying we are now in an active period, which may or may not be related to climate change. What can you tell me?

Yes, hurricane activity varies a lot on decadal timescales, and we entered the current more active phase in 1995. This decadal variability is mainly related to the sea surface temperature (SST) in the North Atlantic relative to the rest of the world - when the North Atlantic is relatively warm there are more hurricanes (I can say more about why that is if you are interested). There is some debate about what drives the SST in the North Atlantic. Some people think it is natural variations in ocean currents, but there is quite compelling evidence that variations in emissions of pollution (anthropogenic aerosols) may have been very important over the 20th century.

2. What does the research tell us to expect in coming decades? I gather that there is some thinking that hurricanes may become more powerful because of climate change but not necessarily more numerous. Is that correct? Can you elaborate on why this is?

If anthropogenic aerosols are important, then hurricane frequency in the coming decades will likely depend on what happens to these emissions. If emissions decrease sharply then the SST in the North Atlantic will be relatively warm (since aerosols have a cooling effect) and hurricane frequency will likely increase. Of course there are many beneficial impacts of reducing aerosol emissions such as improved air quality and less drought in the Sahel. Further ahead (towards the end of the century) models suggest an impact from increasing greenhouse gases, which tend to weaken the tropical circulation leading to fewer hurricanes, although intensity may increase.
27 May 2013


With the flash flooding in San Antonio that killed 3 people, 10" of rain fell in less than 24 hours. What can be said about this disaster's connection to climate change? I'd much rather cite this disaster than the OK tornado, since the link to tornadoes is highly uncertain. Would like to use what you can provide in an oped.


Three Responses:

1) One must be careful with lumping every extreme event into a causality framework but increasingly the peer-reviewed literature indicates that our water cycle is responding to climate change. The 2012 AMS Climate Change statement (http://www.ametsoc.org/policy/2012climatechange.html) notes: "The amount of rain falling in very heavy precipitation events (the heaviest 1% of all precipitation events) has increased over the last 50 years throughout the U.S."

This is also consistent with forthcoming analysis from the National Climate Assessment and our recent paper (Shepherd and Anderson, 2013, Floods in a Changing Climate. See the 3rd figure in Don Wuebble's recent briefing to the Senate (attached document). In our own
analysis of the Atlanta floods of 2009 (Shepherd et al. 2011, Bulletin of the AMS), we also noted that, in addition to increased rainfall intensity, urban areas are particularly vulnerable because of increase impervious surface area. Bottom line, many urban drainage and storm water management systems were designed assuming "stationarity" or that rainstorms of 2013 will be like those of 1950, and we know that is not the case.

Texas has experienced both tails of the hydroclimate extreme in recent years, and generally places that are dry will become drier while places that are wet will likely get wetter. It is important to note that "asking if an event is caused by climate change is probably not the right way to frame the argument." I like to think of it more
from the perspective that climate change is increasing risk or probability of such events as steroids do for a homerun hitter.

2) While no single storm or flood can be attributed to climate change, we know that the incidence of heavy rainfall events has increased during the past 30 years and is expected to increase more in the future as a result of climate change. In other words, the likelihood of such events has increased.

As the climate warms, it allows more water vapor to evaporate into the atmosphere. We have about 4% more water vapor in the atmosphere than we did in 1970. While 4% may not sound like much, storms can gather moisture from large areas. Several studies have noted increases in heavy rainfall, with the biggest increases in the most extreme rainfall events. The most extreme events are becoming more frequent and producing more rainfall.

I agree that you should not use tornadoes as an example. There is no evidence of change in tornado frequency or intensity due to climate change. There are theories about how a changing climate could affect tornado development, but little evidence. Warmer, more humid air might promote development, but weaker winds in the upper atmosphere may inhibit development of tornadoes.

3) Although heavy rainfall events do occur naturally, observations and climate models both indicate that the frequency and intensity of these events increases as the climate warms due to the increase in atmospheric water vapor. So while these extreme rain events are historically unusual, they will become increasingly more common in the future as the climate continues to warm from human emissions of greenhouse gases.
28 February 2013


Hi, all,
I'm working on a story about the Keystone XL pipeline and some of the claims from both sides around it. I wanted to find out:
1. How does carbon from oil sands production compare with carbon from other oil production?
2. Would you agree with what Jim Hansen said, that if KXL is built, it would be \'game over\' for the climate? Why or why not?


Since I know quite a bit about the Keystone, I'll take this. First, I'm sending you a paper on the climate impacts of Keystone. You'll notice that according to this study, if only the "proven" oil reserves are extracted, it would liad to a 0.03 C temperature rise. Of course, the problem is that the Oil In Place, if extracted, would lead to 0.36C rise. This is about half of what we've already observed! And, since technolgoies get better and better for extracting this stuff, it means the proven oil reserves are really a red herring.

A couple of other issues. One byproduct of Alberta Tar sands is PetCoke. This is being used as a replacement to coal and is about 5-10% dirtier than coal.

One issue people raise is that whether we build the keystone pipeline, it will get exported. If not by Keystone, then by some other method. This argument isn't quite true. The alternative route through Canada is facing more opposition that the Keystone pipeline is. So, if Keystone doesn't go through, there is a real extraction problem. Estaimates are that construction of Keystone will increase the extraction by about 35-40%.

So, is it game over? On the one hand it isn't. It would be possible for us to extract Albertan tar sands but then rapidly switch other fuels to low carbon sources so fast that we "make up" for the tar sands. In this case, it isn't game over.

On the other hand, it is game over because this is really the dirtiest of the dirty. If we cannot say no to Alberta, it means we cannot say no to anything. As a consequence, we will be tying ourselves to decades of extracting the dirtiest fuel, at a time when we need to be decreasing emissions, not increasing them. So, in this sense, it is game over.

Personally, I am in the "game over" category. I also believe this is the single biggest climate decision to be made. Cancelling Keystone will send a message that we have a chance to deal with climate chnage. Allowing it to go forward, with a democratic president who has the authority to stop it, means that we will never get off dirty fuels.

One other thing, it is likely the US President will do a quid pro quo. That is, he will allow Keystone to proceed but do something else to clean up the air. This, in my mind, is useless. We cannot avoid climate catastrophy by doing quid pro quos. We need to reduce emissiosn to near zero, not maintain current levels.
19 December 2012


It’s me again. Thank you for your help last time…media surprisingly did not jump on the ‘cosmic rays’ fallacy, but they have seized on something else. The Wall Street Journal’s Matt Ridley is citing Nat Lewis, a reviewer of the IPCC WG1 Scientific Report recently leaked, saying it undermines previous estimates of climate sensitivity.

Here is a link: http://online.wsj.com/article/SB10001424127887323981504578179291222227104.html

Lewis says there is little chance of significant temperature increases by the end of the century, and Ridley concurs: “the observational evidence now points toward lukewarm temperature change with no net harm.”

This is based on Lewis’ claim that the cooling effect of aerosols “hiding” atmospheric warming has been overestimated and is now being adjusted downward, and “the rate at which the ocean is absorbing greenhouse-gas-induced warming is also now known to be fairly modest.” They say these two findings change what we know about climate sensitivity and we can now “[compare] the trend in global temperature over the past 100-150 years with the change in "radiative forcing" (heating or cooling power) from carbon dioxide, aerosols and other sources, minus ocean heat uptake” to arrive at “a good estimate of climate sensitivity.” Ridley says this means that a doubling of CO2 will lead to a warming of 1.6°-1.7°C, lower than IPCC estimates.

Lewis: “Taking the IPCC scenario that assumes a doubling of CO2, plus the equivalent of another 30% rise from other greenhouse gases by 2100, we are likely to experience a further rise of no more than 1°C." He further concludes that a change of “less than 2°C by the end of this century will do no net harm. It will actually do net good.”

Is there a good response to these claims of reduced aerosol impacts or reduced oceanic heat absorption? If not, is there a good response to the temperature projections or their overall effect?


Four responses:

1) When we look at time periods that are only one or two decades in length, the impact of natural climate variations (for example, El Nino and La Nina events) can be very large. This means that climate projections will not pinpoint the actual year-to-year temperature variations over such relatively short periods. In order for the differences between model projections observed temperatures to be meaningful, they must be sustained over more than a few years.

2) Phone interview

3) Oh dear this is a major distraction.

1) It is not an IPCC finding until IPCC is done.
2) Aerosols role is still highly uncertain, especially the so-called indirect effects of aerosols on clouds.
3) One can not estimate sensitivity from observations because the system is not in equilibrium
4) There is very strong evidence that assumptions in simple models are quite wrong: the simple ocean and mixing. On the contrary there is now very good evidence that a LOT of heat is going into the deep ocean in
unprecedented ways, which completely undermines this sort of argument. OHC keeps increasing at a fairly steady rate, just as sea level keeps going up.
5) There is now widespread agreement that stopping global mean T increase at 2 deg C is impossible. Rather 3 deg C will be difficult. See the commentary by Bob Watson at Rio +20 etc.
6) Our science paper does deal with climate sensitivity and we find the only credible numbers are in the upper range.
7) water vapor effects are well established as an amplifier (strong positive feedback)

4) Here is my assessment of the situation. In climate science, you frequently are in the situation where there's lots of data, but the various data tell you different things. That's the situation here. There have been a bunch of publications that have looked at the 20th century temperature record and they almost all conclude that the
climate sensitivity is between 1.5 and 2°C for doubled carbon dioxide. Thus, this new Lewis estimate is really nothing new.

On the other hand, you can look at the paleoclimate data (e.g., ice ages, PETM) and you end up with larger climate sensitivities. You can also construct a climate sensitivity by measuring the values of the individual feedbacks and adding them together. This is what I've done in my research and I get values that are also higher than the 20th century values.

Importantly, each of these estimates has warts and imperfections and thus judgment has to be used in assembling a single estimate of the climate sensitivity. The climate skeptics get their result by simply ignoring the higher estimates and focusing exclusively on the 20th
century record. If you do that, then you get their conclusion. But I don't think that's the scientifically sensible thing to do. There is value in the paleoclimate estimates and the estimates from the feedback calculations.

If you consider all of the data, my judgment is that the IPCC's canonical range gives about the right probability distribution: 2-4.5°C is the likely range, with values below 1.5°C being very unlikely.
13 December 2012


I am a writer and environmental engineer who is doing research for a potential movie that includes an ice mass falling off the Antarctica continent which raises world sea levels by a height of 6 feet.

I’ve read about you and your important work in the field of rising sea levels and would like to ask you (or one of your staff) two questions relating to the subject –

1) How large of an ice mass would have to fall into the ocean to raise the world ocean levels a height of 6 feet? I understand the ice mass on Antarctica is between 1 -2 miles thick, so perhaps we would figure an average height of ice of 1.5 miles if that sounds reasonable. I’ve read that a mass of 5 trillion tons of ice entering the ocean would raise water levels by ½ inch around the world; is this a good formula to use?

2) Where on Antarctica would such an ice mass be most likely to fall?

For this story, we assume that water pooling during the summer months contribute to the event by reducing friction as the ice mass slides off the continent.

I‘ll be glad to provide more details if you would like and we can provide an acknowledgement during the movie credits if desired.


The ice thickness ranges from 0 thickness (any units) to 2.9 miles at the thickest, with the average around the continent being about 1.3 to 1.5 miles as you estimate. Yes, 5 trillion tons (5e12) equates to approximately 0.5 inch global SLR. Although I'm always wary of tons as a unit because there are so many different versions of it. Given 5e12 tons = 0.5 inch, and you want to raise it by 6 feet, that is 6ft * 12in/ft * 1/2in * 5e12ton = 1.8e14 ton or 180 trillion tons. Converting from weight to area, assuming 919 kg/m^3 and converting to the units we're using, I get about 50,000 cubic miles of ice needed for 6 feet of SLR. If the ice is only 1.5 miles thick, that is 33,333 square miles of ice 1.5 miles thick.

There are many places in Antarctica where one can find 33 thousand square miles of ice. The catchment of Pine Island Glacier (PIG) is is about 68,000 sq. mi., so half of PIG could do 6 feet of SLR *IF THE 1.5 MILE THICK ASSUMPTION IS USED*. In reality PIG is less than 1.5 miles thick. PIG is currently the poster-child for Antarctic glaciers accelerating into the sea.

There are very few places in Antarctica where water pools on the surface. This assumption limits your ice source to the far north peninsula region (or a much warmer world). Right now I think the melt ponds in Antarctica are only found at the very lowest (warmest) altitudes, which are ice shelves, already floating, and therefore already having contributed to SLR even if they are still intact. Perhaps some peninsula glaciers develop melt ponds - I am not too familiar with the glaciers in that area. Anywhere that is 1.5 miles high will be too cold to have liquid water on the surface in our current climate.

Conversely, Greenland has many melt ponds on its surface, which do behave exactly as you suggest - The ponds drain down moulins, and rather quickly we see the ice sheet lift up a small amount and accelerate toward the sea.

To give some context for your quest for an ice source for 6 feet of SLR:

* All glaciers around the world excluding Greenland and Antarctica would contribute only about 17 inches to SLR.

* West Antarctica would raise SLR by about 12-15 feet.

So 6 feet would require about half of all of West Antarctica to enter the sea. PIG is in West Antarctica, so PIG and a few neighboring glaciers might provide 6 feet in reality, or much more than that if the 1.5 mile thick assumption is used.

Two final thoughts - even if there isn't surface ponds that lubricate the bed, as the glaciers speed up (and most are speeding up), the frictional heating increases and adds more water, reducing friction, allowing faster flow, in a self-reinforcing feedback loop. There are also sub-glacial volcanoes (in Iceland many, and perhaps one in recent Antarctic history) that could lubricate the bed with meltwater.
2 October 2012


A reader has asked me, for my weekly Fact Checker column, to look into the news that record ice in Antarctica "contradicts global warming trend," as NewsMax put it. I'm looking for a response from a source or sources knowledgeable about Arctic/Antarctic ice.


Two Responses:

1) It is downright ironic that the Antarctic should move, rather suddenly and only briefly, into record extent territory just as the Arctic was pushing into sub-3.5 million km2 extent. It is a bit like having your high school team win their football game while your NFL team loses the
Super Bowl.

These systems not directly connected, and they certainly don't offset each other. The issue is simply that the climate and ocean processes that control summer Arctic ice extent are completely different from the ones that drive the Antarctic. Both systems are being driven in new directions by human-caused changes. At this point, the 'side effects' for the Antarctic are outweighing the effect of warming, primarily because the Antarctic is a colder pole than the Arctic.

(1) Recent evaluations by several authors have shown that *Antarctica is warming*. It has warmed by anywhere from 1 to 4 degrees F overall in the past 50 years. This is based on weather stations, satellite data, climate models, and even the pattern of temperature left in the upper layers of the ice. But Antarctica is a cold cold place - the big difference between it and the north is that, in the north, a 4 degree warming can be the difference between skating and swimming. In Antarctica, it is the difference between one and two layers of long johns..beneath your parka and your fleece.

The big difference is melting: Antarctica, and the winter Antarcic sea ice, just does not do this very much. It is the Achilles Heel of the Arctic. Once you get to the melting point, everything changes, and the trend of warming and ice loss amplifies.

(2) Antarctica's changes -in winter, in the sea ice- are due more to wind than to warmth, because the warming does not take much of the sea ice area above the freezing point in this season. Instead, the winds that blow around the continent, the 'westerlies', have gotten stronger in response to a stubbornly cold continent, and the slowly warming ocean and land to the north. An added effect is the ozone hole -- another human-caused impact, a seasonal one, but one that also acts to change the winds in the far south. (When ozone is not present, it does not absorb energy from the sun. So the atmosphere far above Antarctica is colder than it should be when the ozone is gone. This leads to a tendency to faster westerly winds throughout the atmosphere).

(3) Antarctica's trend is not nearly as large or as clear as the Arctic's; have a look at Antarctica's trend for the austral _summer_ months (for example, February) on nsidc.org/data/seaice_index. You'll need to poke around a bit - you're looking for: Antarctic:Compare Anomalies:SeaIce Extent and Concentration Trends:Southern:February:ExtentTrends (and 'Refresh'). When you get there - it is a jagged sawtooth pattern in which the trend is a lot less than the year-to-year noise. Antarctica recently (2008) experienced both a record high monthly extent and a near-record low monthly extent in the same year.

(4) The best picture I can give you to place this in a proper view is this. The trend for winter ice in Antarctica is to add about 5,000 miles, or an area about like Connecticut, each year. The Arctic summer loss is about 30,000 miles per year, or about an Indiana.

(5) There is a region of Antarctica that is behaving like the Arctic, and (consistent with what I was saying above) it is the warmest area, where melt happens the most: the Antarcic Peninsula. This area is being pushed over that critical frozen/thawed threshold, and is showing a lot of
changes. Small increases in the colder (and more vast) areas of Antarctica are offsetting this trend for the continent as a whole.

I know its way too much, and believe me, life would have been much easier if Antarctica had just tied its football game this season.

2) I am replying to your enquiry about whether record ice in Antarctica "contradicts global warming trend". In fact, the observation of increased snowfall on Antarctica is entirely consistent with the global warming trend - and is what we'd predict using state-of-the-art climate models.

Antarctica is the coldest place on our planet, and one of the driest. It has been exhibiting warming over the past 50 years, just like global average temperatures. As with other continents, Antarctica has shown a varied spatial pattern of warming, with more warming around the edges and some cooling near the center. The most warming has
occurred on the Antarctic Peninsula and West Antarctica, where several ice shelves are. East Antarctica may have cooled slightly near the central region of the continent over the 1980s and 1990s, although the rate of change is barely discernable in this region.

Antarctic currently is so cold that even with increases of a few degrees, temperatures will remain below the melting point of ice. However, the warming of air temperatures over Antarctica, and globally, has meant that the air over the continent has more water vapor, meaning that more precipitation has fallen. This precipitation has fallen as snow. The exact same principle is how a snow gun
operates - add more water vapor to cold air, and produce snow as a result!
22 September 2012


Inquiry: More intense but less frequent precipitation events seems a fairly robust projection of future climate changes. Clausius-Clapyron considerations make the "more intense" portion of this projection plausible to the intelligent layman, but I am looking for a way to make the "less frequent" portion plausible to the same intelligent laymen. Comments in IPCC AR4 WG1 and mention "mean precipitation is constrained by energy budget" do not convey to me a way to describe this projection in a plausible way to the intelligent layman with some scientific background. A plausible "story" would be appreciated.


The rate of increase in water holding capacity of the atmosphere is 7% per deg C (4% per deg F): this is Clausius Clapeyron. It is also why we have rain: because as air rises it expands and cools and can no longer hold the moisture. In places where there is no shortage of moisture, the relative humidity stays about the same and hence the actual moisture changes at this rate. This applies to the oceans and all maritime and coastal regions.

But that has nothing to do with the hydrological cycle: how much moisture evaporates and how much precipitates. This is controlled by how much
energy is available at the surface and also the ability of the atmosphere to get rid of energy released in the precipitation process: latent heat.

In the latter case, if one just dumps heat in the atmosphere above the surface it would stabilize the atmosphere and prevent rain clouds from
forming. The atmospheric circulation instead carries the heat away (warm air rises) and air subsides elsewhere where it can radiate to space. i.e.
for the process to continue one has to get rid of the heat.

In the case of the energy budget: it depends mostly on how much solar radiation (sun) is absorbed. But that does not change much with the
increasing carbon dioxide until the albedo changes and snow melts, but that is regional. However it also depends on clouds. The differences in
clouds are the main reason why there is a 20% spread in global precipitation among all the models. So how cloud changes matters also. They seem to have a small positive feedback. Nevertheless, in terms of the total hydrological cycle changes these effects are small.

So the most important effect on change with increasing carbon dioxide (and other GHGs) is the increasing downwelling longwave radiation that provides
energy for more evaporation. But the biggest part of that is radiated back to space as the planet warms up. The estimates are that for a 1 deg C warming, the net evaporation increases by about 2% from models. Maybe 1.5 to 3%. The models build in all these other effects including the need to get rid of the heat, and hence the spread. The other big factor that can compromise this is pollution (aerosols) that block the sun. That can
offset this entirely, mostly regionally.

So we have overall E and P increasing at 2% per K, and water holding capacity increasing at 7% per K. Something has got to give. Heavy rains tend to go up at the 7% rate or even more because of feedbacks of the extra buoyancy on storms. The precip process dries the atmosphere and so it takes time to recharge at the lower E rate. This means longer dry spells and/or shorter but more intense rainfall events. In some overall sense precip has to be less frequent.
17 August 2012


I'm hoping to get comments on a new report by Marco Tedesco that Greenland's ice sheet has melted more this season than anytime in recorded history. Do you see this as further evidence of climate change? What impact will accelerated melting have on humans / the environment? Do you think the media should be covering this report? (Is it newsworthy?) Thank you!


I absolutely think the media should cover this more, along with the record-breaking pace of Arctic sea ice loss for the summer. This year is a significant step upward in impacts from climate warming seen in the far North - these are not one-offs, but record events topping two decades of generally increasing effects. Moreover, science results are pointing more and more to temperate-latitude effects of the loss. This has been a hell of a summer.

I think the science community 'gets' that the bandwidth of the American (and global) public is pretty packed right now: economy, deficit, and election; and for some the climate discussion must seem old. But these events need to get on people's radar. Our planet is trying to get our

Melting of Greenland, and direct run-off of the meltwater into the ocean is more than half of the total contribution to sea level rise from the ice sheet, which is now running between one-half and one millimeter per year (~200 to 350 Gtons/year). The other half is from faster glacial flow,
and this too is impacted by the increased melt. [As you many know, we've learned that surface meltwater can punch through the ice, flow out in a thin layer beneath the massive ice sheet, and allow it to slide toward the coast more rapidly.]

Far more significant than the melting at the summit (as astounding as that was), the melt run-off this year at lower elevations was stupendous, turning outwash summer creeks into torrents that damanged infrastructure
and brought unusual amounts of silt (glacial flour) into the ocean (I'm uncertain about the ecological effects of this on fisheries or plankton - it might be positive or negative, not sure).

Arctic sea ice extent has taken a sudden dive downward, and will almost certainly set a record this year; 2012 will break the record of 2007, which was nearly equaled in 2011, then 2010, then 2008.... and our comparison period having reasonable data is 1953-present; before that, but for a blip in the 1930s that resembled the late 1990s or early 2000s, we can find no evidence for a period when Arctic ice was this low for several
thousand years.

[By the way, this Arctic melt-off began with rapid snow-melt in northern lands, setting a record in June, and nearly so in May. The previous record? 2011.]

The sea ice cover as a whole is thinning as well as shrinking. Recent studies from a new satellite (CryoSat-2, launched by ESA) show that a downward trend in thickness we were able to detect with a US satellite
(ICESat-1) has continued. An extrapolation of the trend as seen now goes to zero early in the 2020's (for summer minimum ice thickness). What we will likely see (instead of zero) is a summer pack that is confined more to the Canadian side, with scattered patches of ice within the Arctic Ocean. The main impact of this is a much darker (lower albedo) polar region, leading to surface and ocean warming, less ice, less snow, or in other words 'a less polar Pole'.

That is the problem, climatologically: the Pole in the north may be acting a bit less like a pole in climate. With a cold and stable Arctic, airflow tends to be circular around the pole (zonal). What several studies are now saying is that, rather than a spinning bicycle tire, the patterns look more like a slowly turning flower - large loops, that can get fixed
in a certain pattern for longer periods, leading to highly variable conditions in the temperate latitudes. Cold and snowy, or very mild winters; hot/dry, or rainy/cool summers. Highly variable, with frequent extremes. A longer summer and autumn (longer growing season);
hard-to-predict winters. Overall, a warming trend.

Marco Tedesco (Greenland Melt)
Seymour Laxon and Ron Kwok (Sea ice thinning ICESat and CryoSat)
David Robinson (snow cover record in June)
NSIDC ASINA (sea ice trends)
Jim Overland (sea ice loss leads to less polar pole)
Polyak et al (history of sea ice cover in the Arctic)
Jennifer Francis (trend towards more variable temperate weather)
3 July 2012


I'm doing an article for XXXXX about how climate scientists now tend to explain extreme weather events (as seen in US at moment) in the context of climate change. There has always been the caveat that you shouldn't directly link/conflate the two, but I get the sense that there has been some subtle movement on this over the past year of so. (I guess the Russian heatwave in 2010 changed this in terms of AGW attribution etc??)

I want to get the view of a few climate scientists on where they now stand on this. Some, for example, now seem to be trying to explain it with the analogy of increasingly loaded dice. Would you be able to get a few climate scientists to email me a paragraph or two that I can quote verbatim setting out their position on linking extreme weather events with climate change? Some, for example, still might think the caveat mentioned above is still very important to stress. Others now less so?


Seven Responses:

1) There is no evidence that I am aware of weather extremes will decrease with climate change. However, there is a substantial and growing body of evidence indicating that the proportion of intense weather events is likely to increase as the globe warms. The confidence in this statement varies from uncertain, e.g. in the case of tornadoes, to quite confident, as in the published consensus viewpoint that the proportion intense hurricanes will increase substantially.

With regard to current weather extremes, there is a diversity of views. My considered view is that the occurrence of specific systems cannot be attributed to climate change, but that there is an element of increased intensity for those extremes that do occur. For hurricanes, I consider that up to 50% of the observed global and regional increases of intense hurricanes over the past 30 years can be attributed directly to global warming.

There has been a strong and inarguable increase in impacts from severe weather. But this is primarily due to increasing population and migrations to areas more vulnerable to extremes, such as tropical coastal areas.

2) I think it is inevitable that climate change will affect the frequency and intensity of extreme events. Weather can be characterised in terms of a mean value (the climate) and variability around the mean. Climate change will shift the mean value (by definition), and hence change the probability of extremes unless the variability also changes to compensate exactly (and there is no reason to expect this). The difficulty is in calculating the contribution of climate change to an individual extreme event. This is currently an active research area, known as operational attribution, in which many climate model simulations are made with and without forcing due to climate change in order to compute differences in the probabilities of particular events.

3) The link between extreme events which have occurred recently and the buildup of the greenhouse gases is indeed best represented by the loading the dice analogy as the world warms, the likelihood of occurrence (frequency), intensity, and/or geographic extent of many types of extreme events is increasing. The events are individual data points in a broader pattern, akin to pixels on a computer screen. You cant say much from any one pixel but a picture emerges when you step back and look at the pattern. That said, for a few types of extreme events, particularly heat waves, it is sometimes possible to connect the pixel to the bigger picture more directly. The best case is the European heat wave of 2003. According to computer simulations of climate, the likelihood that such an event would occur was about doubled by the buildup of the greenhouse gases. A few other events have been examined using similar techniques, including the 2010 heat wave in Russia.

As for the willingness of scientists to make such statements: as the climate signal due to the ever-increasing greenhouse effect strengthens and emerges more and more from the noise in the system, and as statistical techniques for doing such fingerprinting studies as I mention above improves, scientists have become more confident in making such claims, which is to be expected.

4) I am attaching a brand new summary just put out by some colleagues explaining how these extremes and climate change fit. There has been a key breakthrough in analysis on this topic. Instead of trying to interpret a single event, the climate scientists are studying the frequency distribution of events and seeing if the event, like record heat, is occurring more frequently. Look at Fig 3 in this report for a good example. Climate scientists are now using this analytical technique to study many extreme events. This avoids the trap of overinterpreting any single event, and focuses on frequency of occurrence rather than the magnitude of the individual event.

5) Attribution of extremes is challenging. We're faced with separate, but related, questions. First, how much did the warming of the planet contribute directly to the extreme event? Second, how much more likely was the event because of a warmer planet? For things that are closely
related to temperature (e.g., heat waves. fires), the first question can be addressed in a relatively straightforward manner and, typically, the answers are conservative. Even with a degree or two of global warming, the direct contribution to extreme heat, such as in the southern Plains of the US in 2011 and in much of the US in 2012, is small.

The second question is more challenging to address. There are two issues that need to be considered. First is a statistical problem about how the likelihood of low probability events changes as the average condition changes. For example, if you flip a fair coin 100 times, on average you get 50 heads and 95% of the time, you'll get
between 40 and 60 heads and 2 or 3 times, you'll get 65 heads. If you get a weighted coin that is 55% likely to be heads, it will 10 times as likely that you'll get 65 heads. The small change in the average chance means the chance of an extreme becomes much more likely. The
same thing happens for temperature extremes, but there's another issue. Did the change in the average temperature make it more likely that the flow in the atmosphere was even more likely to occur than just by chance? For instance, when it doesn't rain much over a large area, the ground dries out and heats up. The atmosphere responds to this by flowing around the area of hot air in a way that makes rain even less likely in the hot area, leading to more heating of the ground, reinforcing the flow around that area.

For things that aren't temperature, we have to work to understand the relationship between the global temperature and the phenomenon in question. For instance, we understand that warming the planet will likely lead to a more intense water cycle, with heavier rain when it rains and longer periods without rain in between. On the other hand, our understanding of how global scale atmospheric changes affect things like tornadoes and severe thunderstorms is that global warming will make some of the ingredients for them more likely and others less likely. As a result, it appears that long term trends in tornado occurrence or intensity are unlikely to be large. Even without the planet warming, we would expect to see some years with many tornadoes
and others with few tornadoes.

6) "Whenever an extreme weather event occurs, it is natural for the public to ask, Is this event a result of global warming? This is not the quite the correct question to ask, as to date, all individual weather events observed could have happened prior to the human intervention in the climate system, however unlikely that may have been. The more relevant question to ask is How has the risk of this event changed because of climate change?

This risk of extreme weather, particularly very severe heat waves, has already changed significantly due to human induced global warming. For instance, the chances of the 2003 European summer heat wave, responsible for as many as 70000 additional deaths, at least doubled and likely increased by a factor of 4 to 10. The chances of the 2010 Russian and 2011 Texas events also undoubtedly increased. While these events could have occurred without the human changes to the climate, it is important to know that the amount of climate change that we have experienced so far is very small to what is projected to occur by the middle and end of this century. By 2100, today's most extreme weather events will seem relatively normal."

7) I am responding to your inquiry about linking the recent extreme weather in the U.S. to climate change. In my view, the only responsible statement scientists can make about this regards the probabilities of such events with and without climate change. We should be able to say something like "the annual probability of a heat wave of magnitude A and duration B before the advent of climate change was x but as a result of climate change has increased to y and is expected to further increase to between z1 and z2". It would take some work to actually fill in the numbers x,y, z1, and z2, but there are studies along these lines for events such as the 2003 European heat wave. In my view, any statement that goes appreciably beyond statements like this one probably involves spin of one kind or another.

In addition, once could talk about the particular routes by which climate change affects particular events. For example, the fires in the Rockies have apparently been affected by the ill health of many trees, owing to a population explosion among pine beetles, which is in turn partly owing to climate change.