|Description:||Sceptics are taken to task for criticising evidence that supports global warming and embracing arguments that deny it.|
|Climate change and sensitivity: not all Watts are equal - Tue, 11 Mar 2014 00:40:09 EST|
We hear a lot of talk these daysaboutclimate sensitivity. It is often considered the most important measure for predicting how much the Earth's temperature will increase as we emit heat-trapping gases like carbon dioxide.
But, the term sensitivity has to be used carefully because it can mean different things in different contexts. For instance, there is a long-term (equilibrium) sensitivity to doubling carbon dioxide which refers to the ultimate temperature reached by the planet if we were to double carbon dioxide. There are also shorter term (transient) sensitivities which relate to temperature changes as heat trapping gases increase at some specified rate.
Values for climate sensitivity in general can be obtained many ways. My favorite way is by looking at deep history. If we can measure how sensitive the climate was in the past, perhaps we can infer its sensitivity now. A second way is through the use of modern temperature records and recent greenhouse gas levels. A third way is through the use of climate models (computer programs that replicate the Earth climate system). Regardless of the method used, there is general agreement that if we were to double carbon dioxide, the Earth's surface temperature would eventually increase by 1.5–4.5°C (2.7–8.1°F). Obviously, if the Earth sensitivity is at the upper end of the range, we are in trouble.
Recently, there have been some studies whichsuggest that maybe the climate sensitivity is at the lower end of this range. Most of these studies have only used the second method to calculate sensitivity, a fact that will soon become important. In addition to real science studies, there have been policy organizations that have promoted these low-sensitivity results. But my question is, what does the science say? Fortunately,a paper just published inNature Climate Changeprovides some guidance on this question. The study was completed byDr. Drew Shindell from NASA, and what he found was exciting. It turns out, not all Watts are equal. Energy changes to the Earth system from changes of sun-reflecting particulates or from ozone have a different impact than energy changes from carbon dioxide.
The Earth has a greater sensitivity to particulates and ozone than to carbon. The reason for this seemingly strange behavior is that aerosols are largely located near industrialized areas in the Northern Hemisphere. This hemisphere also happens to contain much more land area than the south– and land regions are more sensitive to changes in energy, at least in the near term. In short, particulates and ozone impact more sensitive parts of the planet. Carbon dioxide, on the other hand, spreads out uniformly across the globe– it doesn't accumulate in one hemisphere or another.
However, let's not get too excited about particulates saving us from global warming. Dr. Shindell also showed that while in the short run, the cooling effect from particulates matters a lot, in the long run, it doesn't make much of a difference. The impact can be seen in this figure which shows the prior expectations of the climate (dashed line) alongside the revised prediction (solid). By 2050, there really is little difference.
What does this have to do with climate sensitivity?
|GWPF optimism on climate sensitivity is ill-founded - Mon, 10 Mar 2014 08:19:21 EST|
The UK anti-climate policy advocacy group Global Warming Policy Foundation (GWPF) has publisheda reportwritten by Nic Lewis and Marcel Crok claiming“the IPCC hid the good news” regarding climate sensitivity. Lewis is an amateur researcher and retired financier who haspublished a few papersestimating climate sensitivity, and Crok is a freelance science writer.
GWPF asked climate scientist Judith Curry to write the Foreword to the report, presumably to lend it more credibility. However, Curry has no publications or expertise in this area,and once saidthat the global equilibrium climate sensitivity could fall anywhere between 0 and 10°C for doubled CO2. This comment is totally incompatible with the body of climate sensitivity research, and also with the GWPF report.
The report itself is essentially a commentary and includes no new information. It boils down to Lewis and Crok trying to make the case that climate sensitivity is on the lower end of the IPCC estimated range. In the report, they find reasons to dismiss the many studies and varying approaches that arrive at higher climate sensitivity estimates, and fail to discuss the shortcomings of the lower sensitivity studies that they prefer. In short, it’s a very selective and biased review of the scientific literature on the subject. Recent papers by Gavin Schmidt and Drew Shindell at NASA GISS, not considered in the GWPF report, entirely contradict its conclusions, for example.
There are a few main methods to estimate the global climate sensitivity; Lewis and Crok focus on three of these and present their case for why each should be considered valid (when yielding low sensitivity results) or disregarded (when yieliding moderate or high sensitivity results). Here we’ll look at each, including evidence the GWPF report failed to consider, and show that their conclusions are not supported when the full body of research is considered. As climate scientist Steven Sherwooddescribed it,
However, the good news is that the report is consistent withthe 97 percent expert consensuson human-caused global warming. It acknowledges that global warming will continue as long as humans continue increasing the greenhouse effect, and merely suggests that future warming will be toward the lower, slower end of the IPCC estimates. As climate scientist Ed Hawkins at the University of Reading also noted,
Paleoclimate studies attempt to estimate climate sensitivity based on the forcings and temperature responses from climate change events in the geologic record. The most robust study of this type was done by the PALEOSENS team,published in Nature in 2012. This study evaluated past climate changes over the previous 65 million years, considering nearly two dozen investigations of many different geological time periods.
The study estimated with 68 percent probability that the equivalent equilibrium climate sensitivity is between 2.2 and 4.8°C for a doubling of CO2, generally consistent with IPCC estimates, and inconsistent with the lower estimates preferred by GWPF. The 95 percent confidence range in this study was between about 1 and 7°C equilibrium sensitivity, so very low and very high climate sensitivities could not be ruled out, but are relatively unlikely, based on the historical record. Additionally, the GWPF report uses 68 percent confidence ranges throughout, so the 2.2 and 4.8°C PALEOSENS paleoclimate estimate is inconsistent with the GWPF low sensitivity conclusions.
Various paleoclimate-based equilibriumclimate sensitivityestimates from a range of geologic time periods. Adapted fromPALEOSENS (2012)Figure 3a by John Cook.
The GWPF report has very little discussion of paleoclimate sensitivity estimates. They just say that these studies don’t tightly constrain the possible climate sensitivity range, and past climate states are different than current and future climate states, so“little weight can be put on the palaeoclimate estimates.” While there is some truth to these critiques, entirely disregarding the results of these studies is simply not justifiable.
In summary, paleoclimate studies provide one line of evidence that supports an equilibrium climate sensitivity between about 2 and 4.5°C, and the GWPF justification for dismissing these estimates is weak.
General Circulation Models
Climate models (general circulation models or GCMs) provide another method by which to estimate climate sensitivity. The physics of the climate system are input into very detailed climate models, which can then estimate how the global temperature will respond to various forcings. The results can give us projections of future global warming under a variety of scenarios, and also give us an estimate of the global climate sensitivity. Most GCM equilibrium climate sensitivities range between 2 and 4.5°C (average 3.2°C in GCMs used in IPCC AR5). This range is consistent with paleoclimate estimates.
Lewis and Crok make the following argument.
However, according to climate modeler Gavin Schmidt of NASA GISS, this is incorrect.
Climatescientist Kevin Trenberth also notes that the change in the estimated aerosol forcing is mainly associated withindirect aerosol effects, but half of GCMs don’t include these indirect effects, and those that do actually tend to simulate less warming.
This point was also made bySchmidt et al. (2014), which additionally showed that incorporating the most recent estimates of aerosol, solar, and greenhouse gas forcings, as well as the El Niño Southern Oscillation (ENSO) andtemperature measurement biases, the discrepancy between average GCM global surface warming projections and observations is significantly reduced. This approach also accounts for the previously underestimated volcanic aerosol forcing, demonstrated bySanter et al. (2014), but not included in the GWPF report.
GCM mean (dark blue #1) and envelope (lighter blue) range of global surface temperature projections vs. HadCRUT4 (red #1) andCowtan& Way(red #2) global surface temperature instrumental estimates. The GCM mean result incorporating changes in ENSO and updated solar and aerosol forcings (blue #2 and #3) are also shown. Adapted fromSchmidt et al. (2014)by Kevin Cowtan.
In summary, GCMs provide another line of evidence that generally supports an equilibrium climate sensitivity between about 2 and 4.5°C, and the GWPF justification for dismissing these estimates is incorrect.
The method preferred by the GWPF report, and that which Lewis has used in his own papers, involves estimating climate sensitivity using a combination of recent instrumental temperature data (including ocean heat content data), less complex climate models, and statistics. A few studies using this approach since about 2012 have begun yielding lower climate sensitivity estimates. In their report, GWPF citeRing et al. (2012),Aldrin et al. (2012),Lewis (2013), andOtto et al. (2013)as all yielding central equilibrium climate sensitivity estimates between 1.76 and 2.00°C. However, the GWPF report only references the“main results” of Aldrin et al. (2012), whose study actually estimated equilibrium climate sensitivity of about2.5 or 3.3°C when accounting for cloud and indirect aerosol effects. Aldrin et al. wrote,
The GWPF report, however, did not interpret their estimate with care. It simply used the result that was convenient for their argument, and left out the cloud uncertainties.
As for Lewis (2013), it's not without its own red flags. As documented at And Then There's Physics, when using data up to 1995, the method yields an estimated climate sensitivity range of 2.0–3.6°C, but incorporating an additional 6 years of data reduces the estimate approximately 33 percent, to 1.2–2.2°C. Climate sensitivity is a relatively constant parameter; if adding just 6 years of data changes the result so dramatically, one should really question the method being used. Instead, Lewis argues that it's the only reliable method for estimating climate sensitivity.
The challenge with this 'instrumental' method of estimating equilibrium sensitivity is that it’s based on transient instrumental measurements. There is currently a global energy imbalance, and reaching a new equilibrium state will take over a century. Therefore, estimating equilibrium climate sensitivity based on measurements of a climate that’s out of equilibrium requires making some significant assumptions, for example that feedbacks will remain constant over time. However, several recent studies have suggested that these assumptions may not be correct. For example,Armour et al. (2013),
Trenberth and Fasullo (2013)also note that ocean heat content (OHC) variability can strongly impact the 'instrumental' climate sensitivity estimates (emphasis added).
None of these papers or concerns with‘instrumental’ climate sensitivity estimation methods are mentioned in the GWPF report. Instead, the report argues that this approach provides the only reliable method for estimating climate sensitivity, and that all other methods that produce higher estimates (e.g. paleoclimate and GCMs) are wrong.
However,an important new paperjust published by Drew Shindell at NASA GISS reconciles the difference between the climate sensitivity estimates in these varying approaches, but not in the direction advocated by the GWPF report. Shindell notes that the‘instrumental’ approach studies preferred by the GWPF report assume that the global mean temperature response to all forcings is equal. His study investigates this assumption by comparing GCM temperature responses to greenhouse gases with their responses to aerosols and ozone.
Shindell, who was a co-author on Otto et al. (2013), notes that“forcing in the NH extratropics[above 30° latitude]causes a greater global mean temperature response than forcing in the tropics”; a result noted byHansen et al. (1997):
The forcing from aerosols and ozone isn’t globally uniform, but instead focused more in the northern hemisphere extratropics. Hence it results in a relatively larger temperature response than an equivalent forcing from greenhouse gases, which are well mixed throughout the atmosphere.
When assuming equal sensitivity to all forcings, Shindell estimates the transient climate response (TCR) at 1.0–2.1°C, most likely 1.4°C, which is almost identical to the Lewis GWPF report estimate (1–2°C, most likely 1.35°C) and also similar to the estimate in Otto et al. (2013). However, when Shindell accounts for the higher sensitivity to the aerosol and ozone forcings, the estimated TCR range rises to 1.3–3.2°C, most likely 1.7°C. Compared to the IPCC estimated TCR range of 1–2.5°C, and the range in climate models of 1.1–2.6°C, Shindell's results give a low probability for the low end of the range and higher probability for the high end; the opposite of the GWPF report. Given the strong correlation between TCR and equilibrium climate sensitivity, Shindell’s results also suggest that the lower climate sensitivity estimates are unlikely to be accurate.
Accounting for Cloud and Water Vapor Observations
The GWPF report also notes that changes in cloud cover in a warming world are a key to determining climate sensitivity. On this topic the report merely claims“Observational evidence for a positive cloud feedback is weak, at best.” However, there have been several studies comparing observed changes in cloud cover to cloud simulations in climate models. For example,Fasullo and Trenberth (2012)used satellite data from the NASA Atmospheric Infrared Sounder (AIRS) and Clouds and the Earth's Radiant Energy System (CERES) to examine the relationship between seasonal changes in relative humidity (RH) in the dry subtropics and the Earth's albedo via cloud cover.
Sherwood et al. (2014)built on the work of Fasullo and Trenberth by looking at the way that variousclimate models handle thecloud feedback. They found GCMs with a lowclimate sensitivitywere inconsistent with observations. It turns out that these models were incorrectly simulating water vapor being drawn up to higher levels of theatmosphereto form clouds in a warmer world. In reality (based on observations), warming of the loweratmospherepulls water vapor away from those higher cloud-forming levels of theatmosphere,and the amount of cloud formation there actually decreases. The diminished cloud coverleads to greater warming (a positivefeedback), and is better reproduced in the GCMs with higher climate sensitivities.
These studies were omitted from the GWPF report, but they provide yet another line of evidence for high and against low climate sensitivity.
Climate Policies are Insufficient in Any Case
The GWPF report concludes by complaining that by not emphasizing the lower climate sensitivity estimates, the IPCC has“inadequately informed” policymakers about the state of the science. However, from a policy standpoint, we're not doing nearly enough to reduce emissions even in the best case scenario. As Myles Allen, co-author on Otto et al. (2013) noted of the GWPF report,
Moreover, as detailed above, the full body of scientific evidence suggests that climate sensitivity is relatively high. Even if you believe the GWPF report is right, there's a good chance it's not. Proper risk managementtherefore mandates that we must take action to mitigate the threat of dangerous climate change.
But in any case, the full body of evidence is firmly against the conclusions of the report. The authors merely dismiss or ignore the research that doesn’t support their desired conclusion, and overlook the shortcomings of the research that does.
Note:the discussion of the Shindell results has been incorporated intothe Advanced rebuttal to the myth 'climate sensitivity is low'.
|2014 SkS Weekly Digest #10 - Sun, 9 Mar 2014 06:20:50 EST|
If you like the Sunday funnies, you'll love, Cartoon: theclimatecontrarian guide to managing risk,cartoon by John Cook and text by Dana and John Cook. Needless to say, it generated the most comments of the articles posted on SkS during the past week.The Extraordinary UK Winter of 2013-14: a Timeline of WateryChaos by John Mason drew the second highest number of comments. Coming in third wasThe Editor-in-Chief of Science Magazine is wrong to endorse Keystone XL by Andy Skuce.
Toon of the Week
Quote of the Week
"We expect that doubling CO2 and doubling it again will cause dramatic climate change for the same reason we expect day to be warmer than night, summer warmer than winter, and Miami warmer than Minneapolis. It’s not because of computer models, and it’s not because this January was the fourth warmest on record. It’s becausewhen you add heat to things, they change their temperature."
Cause and Effect, by Scott Denning, Climate Change National Forum. Mar 2, 2014
SkS in the News
Confessions of a MOOC Addictby John Cook is psted on the Huffington Post.
In Navigating Climate Science Denialism: Resources for youGreg Laden touts both the SkS website andThe Debunking Handbookvia the article, Debunk me!“Lean, mean and easy to read…”.. Laden also links toThe Consensus Project (TCP)in his postDenying Climate Science in Multiple Dimensions.
Doug Graig links to both theThe Consensus Project (TCP)website and toQuantifying the consensus on anthropogenic global warming in the scientific literature in his blog post,Human fingerprint of climate change.
Five proven strategies to bust unsubstantiated mythsby John Cook& Kate Hannah is posted on the Global Cliamte Intsitute website.
In his Huffington Post article,Through the Climate Portal: Humanity's Tragic Flaw, David Golstein links to theHirsoshima Widget.
Denise Robbins links to the SkS rebuttal,Over 31,000 scientists signed the OISM Petition Projectin her Media Matters for America article,Who Is Patrick Moore? A Look At The Former Greenpeace Member's Industry Ties And Climate Denial.
In his All Voices blog post,Volcanic eruptions contribute to global warming 'pause', Rober Myles cites Dana's SkS article,Does the global warming 'pause' mean what you think it means?
TheGlobal Change Institute (GCI) of Queensland Universitycontributes to evidence-based, progressive solutions to the problems of a rapidly changing world within the existing and projected frameworks of those problems: political, environmental, social, economic, technical.
The University of Queensland has established leadership in many of the issues associated with global change, and is positioned to provide national and international leadership in these areas. The GCI will provide a vehicle for collaborative research, learning, engagement and advocacy in major global change issues.
SkS Week in Review
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