Climate Feedback

Kevin Trenberth

The recent Asian Pacific Economic Cooperation (APEC) summit has brought further attention to climate change and what, if anything, to do about it. In spite of the IPCC findings that global warming is “unequivocal”, doubt remains in some quarters about the reality of climate change and the human cause. Issues are continually raised that have no basis, as highlighted by the recent Commentary from Syun-Ichi Akasofu in the Wall Street Journal.

Akasofu immediately starts out on the wrong foot by claiming there are two sides, those of believers and non-believers, but it is not a matter of belief, it is a matter of scientific facts! He further fails to understand the nature of the IPCC process and the extensive reviews in which all comments are addressed rigorously as the report is developed over three years. Contrary to the claim, there is no assumption by IPCC that recent warming was due to the increased greenhouse effect from increasing human produced carbon dioxide. Rather, climate models that run with and without the human-induced changes in atmospheric composition demonstrate that human warming has emerged from natural climate variability since about 1970.

Over the past 500,000 years or so, temperatures, carbon dioxide and methane have gone up and down more or less in tandem through the major ice ages and interglacial periods, as shown in ice cores. As detailed in a Real Climate blog post on this topic, in the absence of human intervention, these changes happen over time, but not at the rate at which CO2 is currently increasing in the atmosphere. Scientists know that carbon dioxide and methane changes follow rather than cause these changes in temperature between glacial periods, but they also know that these changes in greenhouse gases amplify a relatively weak forcing to help drive temperature change. To suggest otherwise, as Akasofu does, is misleading at best.

Akasofu then trots out the mistaken view that the “hockey stick” curve of temperatures over the past 1000 years showing an upward bend at the end has been discredited. In fact, it has been reinforced in the latest IPCC report, although it is given less emphasis as it is now backed up and confirmed by evidence from multiple independent studies.

He further claims that natural climate variations have been forgotten and attributes recent warming to the “rebounding effect from the little ice age”, but fails to realize that natural climate variability also has a cause. While it is true that we do not have the measurements to show what was happening in the ocean during this time, for instance, we have good reason to believe that natural internal variability played a role. To the extent that the "Little Ice Age" and "Medieval Warm Period" can be meaningfully defined, there has been much work showing that the main variations can be explained in terms of the response of the climate system to natural variability in solar and volcanic events that would have influenced surface temperature. And warm periods in the past, such as the warming in the Greenland region in the mid-twentieth century, were not global in contrast to recent warming, which is.

Climate models are not perfect, but they are useful tools for quantifying the effects of various climate processes and drivers of climate change. Akasofu decries the confused state of climatology, but it is he who is really confused, and his article only serves to confuse the general public. It is sad that a once distinguished newspaper published such misleading half-truths without verifying them.

Kevin Trenberth

Posted by Olive Heffernan on September 19, 2007
Categories: Climate Science, Communicating Climate Change, In the News, Kevin Trenberth | Permalink | Comments (39) | TrackBacks (0)

Kevin Trenberth

The situation on how hurricanes have changed and will likely change in the future are outlined in my recent Scientific American article, but may seem as murky as ever to the public although clarity is actually emerging.

A recent news report in the Press Register outlines some sources of confusion related to just how well the past record is known. It cites work by Landsea that relied on numbers of land-falling storms as a way to calibrate the Atlantic hurricane record, and which concluded that there may be an undercount of 3.2 storms per year prior to 1966. The most recent Eos Transactions has two articles that follow up and point out why use of land-falling storms is misleading [Holland 2007; Eos 88 (36) 348-349] and that the conclusions of increased activity do not change anyway [Mann et al., 2007; Eos 88 (36) 349-350].

The Holland article points out that there are good reasons why the fraction of storms making landfall should change, both because of natural variations and especially if the climate changes. The Mann et al. article adopts the Landsea-suggested changes for the past as a “what if” test and goes on to show that even a substantial underestimate of early 20th century storms does not change the significance of the increase in activity since 1994. Nor does it change the strong relationship with observed sea surface temperatures (SSTs) in the region; the SSTs have a much more reliable observational record and have clearly increased.

Surprisingly, none of these studies refers to what seems to me to be the most definitive analysis of the likely missed storms in the historical counts by Chang and Guo (2007) in which they analyze in detail the actual ship tracks in the past compared with modern tropical storm tracks. To quote their main conclusions: “It is estimated that the number of tropical cyclones not making landfall over any continent or the Caribbean may have been underestimated by up to 2.1 per year during 1904–1913, with this number decreasing to 1.0 per year or less during the 1920s and later decades. Our results suggest that the characteristics of North Atlantic tropical cyclone track statistics might have changed during the 20th century.”

In 2007 the tropical storm season has been fairly normal in many respects up to now. Only 3 hurricanes have been recorded (versus average 3 to 4) but two were category 5 storms, and that is highly unusual. Forecasts of hurricane activity by NOAA and Bill Gray continue to forecast substantially above normal activity in the Atlantic. To me, observing the events thus far, the incredibly intense convective activity in the Indian Ocean from May to July was an important and totally overlooked factor. The subsequent heavy rains and flooding in India and China were no doubt related. The fact that Atlantic hurricane activity is influenced by events in the Indian Ocean seems to be overlooked by the hurricane forecasters.

Kevin Trenberth

Posted by Olive Heffernan on September 17, 2007
Categories: Climate Science, Hurricanes, Kevin Trenberth | Permalink | Comments (3) | TrackBacks (0)

Kevin Trenberth

A recent paper published by Gaertner et al. in Geophysical Research Letters (GRL) (in press as of 12 July 2007) explores the possibility for tropical cyclones to occur in the Mediterranean area in the future with global warming. It has been featured in the Times. The paper correctly points out that tropical cyclones have recently ventured into some unusual places with Catarina on the east coast of Brazil in March 2004 and Vince making landfall in Spain in 2005. Gaertner et al. use an ensemble of regional climate models to assess new locations of tropical cyclone occurrence. They find an increase in extremes of cyclone intensity over the Mediterranean Sea in regional model climate change scenario simulations. This increase, they claim, is clearly related to tropical cyclone formation, revealing for the first time a risk of tropical cyclone development over the Mediterranean Sea under future climate change conditions.

The regional model framework for this study is one limitation as it may not create the correct large-scale atmospheric circulation across the region. In particular, the Mediterranean climate in summer is one of clear skies and sunny days associated with the overall global monsoon circulation, such that the upward motion and rains in southern Asia are linked to the subsiding air over the Mediterranean that makes for a very stable environment unfavorable for storms. Unless that link is properly simulated (and it may not be, especially in a regional model) the vertical atmospheric temperature and wind structure are unlikely to be right.

The authors are aware of the summer difficulties and so they choose September as the time for the simulations. This has the advantage that the sea surface temperatures (SSTs) are highest then. The sun has been beating down all summer, but it tends to form a shallow layer of warm water and whether there is adequate heat below the surface is also a critical question. As cyclones form they churn up the ocean, bringing cold waters up from below and cool the ocean, creating a cold wake. In this work the SSTs were specified and fixed and this was not allowed to happen.

Moreover, by September, the mid-latitude westerlies have set in and the winds in the upper troposphere are typically over 20 m/s, so the wind shear environment is generally unfavorable for such storms. It is possible that the weather could change enough to relax the winds for just long enough to open a window for tropical cyclones but the odds are not high. This is the extratropics, after all.

All these questions remain. Nonetheless, with higher SSTs it does seem likely that cyclones would be more vigorous. But whether they are truly tropical or not is a key issue. The paper does not comment on water vapor: the Mediterranean Sea is surrounded by land where the air is drier and this can cause storms to run out of moisture and peter out. Nor does it mention rainfalls and flooding: a chief characteristic of tropical cyclones is the heavy rainfalls of several mm per hour. Nonetheless, warm core storms have been noted in the Mediterranean and are colloquially referred to as “Medicanes”.

For further discussion of tropical cyclones and climate change, see my recent article in Scientific American here.

Kevin E. Trenberth
Head of the Climate Analysis
US National Center for Atmospheric Research

Posted by Olive Heffernan on July 24, 2007
Categories: Atmospheric Science, Climate Science, Hurricanes, In the News, Kevin Trenberth | Permalink | Comments (0) | TrackBacks (0)

Posted by Olive Heffernan on behalf of Kevin Trenberth

Given that human induced climate change is with us, a looming challenge is to predict just what the climate will be. To date, there are no such predictions although the projections given by the Intergovernmental Panel on Climate Change (IPCC) are often treated as such. The distinction is important. A paper presented at the International Forecasting Symposium in New York City in late June 2007 by J. Scott Armstrong and K. C. Green is highly critical of IPCC procedures and "forecasts" for not being based on "evidence based" procedures as outlined in an earlier 2001 book of his. It is true that IPCC does not refer to Armstrong's work as it has dubious relevance.

In fact IPCC does not do forecasts, as explained in my earlier post. The IPCC instead proffers "what if" projections of future climate that correspond to certain emissions scenarios. Armstrong has evidently read only chapter 8 of the IPCC Working Group I report and has therefore overlooked the fact that the other chapters address many of the things he is critical of.

In particular there is clear evidence (“warming is unequivocal”) that climate is changing in ways consistent with the climate forcings. Also, the projections are for all aspects of climate, not just global mean temperature. It has been said that “All models are wrong, but some are useful”. The Armstrong forecast of no change is not useful when the system is already changing in ways consistent with human influences on the composition of the atmosphere. Nonetheless, improvements in forecasting procedures are always welcome.

Bob Carter, a climate change doubter in Australia, has written a distortion of all this in the Courier Mail, issuing various attack against the science of climate change. Andrew Ash has written a rebuttal of these comments.

Another key point is that unlike forecasts based on past experience, weather forecasts are based on numerical weather prediction models and rigorous procedures, not empirical methods, although the latter are used to provide added value (e.g., based on known biases in the model). My own presentation at the same conference provides a description of weather and climate prediction.

Continue reading "Global Warming and Forecasts of Climate Change " »

Posted by Olive Heffernan on July 11, 2007
Categories: Climate Science, Conferences and Meetings, Kevin Trenberth | Permalink | Comments (13) | TrackBacks (0)

Posted by Olive Heffernan on behalf of Kevin E. Trenberth

Aerosols in the atmosphere have been hot topics in several recent climate studies but one wonders if the pollution has made not only the atmosphere murkier but also the scientific reasoning?

In March a widely reported study in PNAS by Zhang et al. linked changes in storm tracks over the North Pacific to Asian pollution. In this case effects on radiative forcing by atmospheric aerosols supposedly increased deep convective clouds over the Pacific Ocean in winter, a finding based on long-term satellite cloud measurements (1984–2005). The blame was assigned to the aerosol effect from Asian pollution, which supposedly leads to intensified storms. Unfortunately the authors did not seem to be aware of the major problems in the satellite-based cloud data that give spurious trends owing to changes in satellites and associated instruments. In particular there were major changes in 1994 in all 3 geostationary satellites (GOES West, GOES East and GMS (Japanese)) observing the Pacific Ocean. All are known to be associated with spurious changes in cloud present in the International Satellite Cloud Climatology Project (ISCCP) data. The ISCCP record is simply not reliable for trends. The time series of the paper are perhaps more a measure of the problems with the data than they are of climate change or evidence of effects of aerosols?

Another recent study by Lau and Kim in the February 27 issue of EOS claims that nature foiled the 2006 hurricane season by increasing aerosols over the tropical North Atlantic in the summer of 2006 (see also Wu 2007 for more general analysis). There is no doubt that sea surface temperatures were lower in 2006 than in the record breaking year of 2005 and that aerosols were more plentiful. It is also clear that there were fewer tropical storms in 2006. But is the former the cause of the latter, as inferred in the article? Or is it more likely that the absence of storms led to less rain and thus less washout of aerosols leading to the increase in atmospheric aerosols? Of course, even in the latter case, there may be a feedback, but estimates of the aerosol effects places them much smaller than effects from changes in cloud and sunshine and from evaporative cooling as winds change.

[Note the contrasting findings between the above two studies with one finding more convection and the other finding less, all due to aerosols!]

Now someone else thinks they can use aerosols to change hurricanes, (as reported by Joshua Zaffos in the Rocky Mountain Chronicle in June 2007). Bill Cotton, a professor from Colorado State University, thinks that dumping dust into a hurricane could be used to weaken the storm. The irony in this case is that Cotton is a skeptic on the issue that humans are changing the climate in spite of overwhelming evidence. Human changes in the greenhouse effect amount to about a 1% change in the energy flow through the climate system. But the energy in a hurricane is enormous; it is typically equivalent in energy to a 1 megatonne nuclear bomb going off every 5 seconds, and Cotton thinks he can mess with a hurricane! What if he is actually right and instead it strengthens the storm?

Posted by Olive Heffernan on June 26, 2007
Categories: Atmospheric Science, Climate Science, Kevin Trenberth | Permalink | Comments (2) | TrackBacks (0)

Posted by Oliver Morton on behalf of Kevin E. Trenberth

I have often seen references to predictions of future climate by the Intergovernmental Panel on Climate Change (IPCC), presumably through the IPCC assessments (the various chapters in the recently completedWorking Group I Fourth Assessment report ican be accessed through this listing). In fact, since the last report it is also often stated that the science is settled or done and now is the time for action.

In fact there are no predictions by IPCC at all. And there never have been. The IPCC instead proffers “what if” projections of future climate that correspond to certain emissions scenarios. There are a number of assumptions that go into these emissions scenarios. They are intended to cover a range of possible self consistent “story lines” that then provide decision makers with information about which paths might be more desirable. But they do not consider many things like the recovery of the ozone layer, for instance, or observed trends in forcing agents. There is no estimate, even probabilistically, as to the likelihood of any emissions scenario and no best guess.

Even if there were, the projections are based on model results that provide differences of the future climate relative to that today. None of the models used by IPCC are initialized to the observed state and none of the climate states in the models correspond even remotely to the current observed climate. In particular, the state of the oceans, sea ice, and soil moisture has no relationship to the observed state at any recent time in any of the IPCC models. There is neither an El Niño sequence nor any Pacific Decadal Oscillation that replicates the recent past; yet these are critical modes of variability that affect Pacific rim countries and beyond. The Atlantic Multidecadal Oscillation, that may depend on the thermohaline circulation and thus ocean currents in the Atlantic, is not set up to match today’s state, but it is a critical component of the Atlantic hurricanes and it undoubtedly affects forecasts for the next decade from Brazil to Europe. Moreover, the starting climate state in several of the models may depart significantly from the real climate owing to model errors. I postulate that regional climate change is impossible to deal with properly unless the models are initialized.

The current projection method works to the extent it does because it utilizes differences from one time to another and the main model bias and systematic errors are thereby subtracted out. This assumes linearity. It works for global forced variations, but it can not work for many aspects of climate, especially those related to the water cycle. For instance, if the current state is one of drought then it is unlikely to get drier, but unrealistic model states and model biases can easily violate such constraints and project drier conditions. Of course one can initialize a climate model, but a biased model will immediately drift back to the model climate and the predicted trends will then be wrong. Therefore the problem of overcoming this shortcoming, and facing up to initializing climate models means not only obtaining sufficient reliable observations of all aspects of the climate system, but also overcoming model biases. So this is a major challenge.

Continue reading "Predictions of climate" »

Posted by Oliver Morton on June 04, 2007
Categories: Climate Science, Climate variability, Communicating Climate Change, Kevin Trenberth | Permalink | Comments (12) | TrackBacks (0)

Posted by Olive Heffernan on behalf of Kevin Trenberth

The 2007 hurricane season is about to get officially underway. Never mind that nature has already provided the first named storm in the North Atlantic: Andrea. Several forecasts suggest that the 2007 season in the North Atlantic will be well above average. Sea surface temperatures (SSTs) are above normal and atmospheric conditions look likely to be favorable for tropical storm activity.

In 2005, the record breaking year in the North Atlantic, record high SSTs in the critical region from 10 to 20 degrees N in the North Atlantic provided ample fuel for the 28 named storms (normal about 15, previous record 21 in 1933) and 15 hurricanes (normal about 6, previous record 12). Atmospheric conditions were favorable with weak wind shear (that otherwise tends to tear a vortex apart) and the absence of stable layers that prevent convection from developing. In contrast in 2006, SSTs were much lower and closer to the long term normal, and the atmospheric conditions were not favorable, as a developing El Niño in the Pacific created an atmospheric circulation that increased the wind shear in the Atlantic. This year, in 2007, there is no El Niño in the Pacific, and SSTs and the upper ocean heat content are more favorable for Atlantic storms.

There seems to be general agreement on these points, yet the whole issue of Atlantic hurricanes is mired in controversy over the role of global warming. It is not a disagreement that SSTs are higher but rather whether the warming is due to natural processes such as the Atlantic Multi-decadal Oscillation or global warming. To the public, the result is the same for now. To me this is obvious: global warming is “unequivocal” to quote the recent IPCC Working Group I report and global SSTs have increased about 0.6 degrees C. In the last half century this warming is associated with increased greenhouse gases in the atmosphere. It is not possible that the Atlantic has escaped from this warming.

The latest shot fired on this issue was by Chris Landsea in Eos on the long term Atlantic hurricane record. The Atlantic has the best record of tropical storms owing in part to aircraft surveillance since 1944. It is widely recognized that the older records are less reliable as the reports depend on shipping or landfall of the storms. Landsea makes this case quite well, but he goes further. He assumes that the percentage of landfalling storms should be constant over time and this provides a basis for adjusting the older records and increasing the numbers of storms by 2.2 per year from 1900 to 1965, when the satellite era began. The critical assumption is that this fraction should remain the same, yet it is abundantly clear that SSTs have increased and so other things have not remained the same. Given the dependence of hurricanes on high SSTs above about 26 degrees C, shouldn’t increased SSTs increase the scope for storms to develop farther to the east over what were colder waters and for the tracks to become longer? An example may be Vince in October 2005, which was the first tropical storm to make landfall in Spain and Portugal. Indeed shouldn’t one expect a decreasing fraction of landfalling storms in a warming climate?

Continue reading "Global climate change and hurricanes" »

Posted by Olive Heffernan on May 16, 2007
Categories: Climate Science, Kevin Trenberth, Ocean science | Permalink | Comments (3) | TrackBacks (0)