The University of Texas at Austin
  • Climate models get the basic patterns right

    By Marc Airhart
    Marc Airhart
    Published: Nov. 10, 2010

    Myth No. 3: You can’t trust climate models because they do a lousy job representing clouds and aerosols.

    Climate models line graph

    When researchers remove greenhouse gases from the climate models, the observed warming over the last century (in black) evaporates. The red line represents model output with greenhouse gases, the blue line represents model output without. Graph: Stott et al. 2006b

    Climate modelers have traditionally had a hard time incorporating clouds because clouds are very complex. On the one hand, by reflecting sunlight, they tend to cool Earth. On the other, they tend to hold in heat from the surface, which is why cloudy nights tend to be warmer than clear nights. The models also divide the atmosphere up into blocks much larger than clouds, so it’s difficult to create realistically sized clouds.

    Aerosols — microscopic particles produced by volcanoes, burning fossil fuels and other sources — are a challenge to model too. They tend to have a cooling effect because they reflect sunlight and can also form the seeds for reflective water droplets. Perhaps the models are missing important effects from clouds and aerosols that would counteract the effects of greenhouse gases on global temperatures.

    Charles Jackson acknowledged that climate models do have uncertainties and don’t create perfect predictions about future climate. But despite their shortcomings, when used to simulate past climate, the models get the basic patterns correct. The differences tend to come in the amplitudes, not the general patterns.

    One example is the pattern of global temperature increase of the past century. Jackson noted that when researchers remove man-made greenhouse gases from the climate models, leaving behind all the known natural sources of variability, the observed warming over the last century evaporates.

    Another type of pattern is based on how temperatures change over time with respect to depth in the atmosphere and latitude. This is sometimes represented in color coded charts with latitude running along the bottom and depth in the atmosphere running up the side. Red indicates warming and purple indicates cooling. Each major driver of climate (solar activity, volcanoes, greenhouse gases, ozone and sulfate aerosols) has its own unique fingerprint on this kind of image. These fingerprints have been revealed by running the climate models and changing just one forcing while holding the other forcings constant. The fingerprint for greenhouse gases turns out to be very distinct from all the others — warming of the upper troposphere near the equator and cooling in the upper atmosphere. Rong Fu later showed how this distinct fingerprint of greenhouse gasses shows up loud and clear in direct observations of the atmosphere from satellites, indicating that these gases are playing a major role in climate change.

    Fingerprints graph

    Each major driver of climate leaves its own unique fingerprint on atmospheric temperatures. The fingerprint for greenhouse gases turns out to be very distinct from all the others — warming of the upper troposphere near the equator and cooling in the upper atmosphere. Red indicates warming and purple indicates cooling. View a larger version of this graph.Graph: Santer et al. 2003

    Other patterns that climate models accurately recreate in past climate include warming of the oceans, rising sea levels, decreased sea ice and snow cover, retreating ice sheets and glaciers, changes in atmospheric circulation patterns, and increased heavy precipitation. The fact that climate models get these patterns right gives modelers some confidence in their forecasts of future climate.

    Read the other myths in this series.

    You are invited to post comments and follow-up questions on this site. You can also e-mail climate scientists questions. The scientists cannot respond to all questions individually but will address recurring themes with new entries.

    • Quote 2
      Marc Airhart said on Oct. 12, 2011 at 2:43 p.m.
      Ian, thanks for writing. If we had covered a 9th climate misnomer it might have been titled: "Climate scientists in the 1970s warned us of impending global cooling." Thomas Peterson and colleagues surveyed the scientific, peer-reviewed literature from 1965 to 1979 and found that only 1 out of 6 papers supported the idea that Earth was experiencing global cooling. Peterson et al. conclude that "global cooling was never more than a minor aspect of the scientific climate change literature of the era, let alone the scientific consensus..." Here's their 2008 paper in the American Meteorological Society's journal:
    • Quote 2
      Rick said on Oct. 12, 2011 at 7:08 a.m.
      "The fact that climate models get these patterns right gives modelers some confidence in their forecasts of future climate." But it is a hindcast in a model with many free parameters. Fiddle the parameters until you get a fit doesn't mean that you have actually understood the nature of climate. Science is about testing a hypothesis (e.g. a climate model) against data. If you look at the predictions of the "mean of models" as the IPCC prefer, then it is glaringly obvious that they are not correct, neither in aggregate nor in spatial distribution. The above cited statement is a bad argument.
    • Quote 2
      Ian Parmenter said on June 20, 2011 at 9:44 a.m.
      Why have we had global cooling theories in the 70s that then changed to global warming that then changed to the hole in the ozone & cfcs, that then changed to co2? Somewhere someone is not getting it right.
    • Quote 2
      burgess and burgess said on Jan. 6, 2011 at 4:53 p.m.
      I think everything about the climate is unpredictable no matter what model you use. Maybe that's why I like it so much. Thanks for the article!
    • Quote 2
      Marc Airhart said on Dec. 14, 2010 at 12:43 p.m.
      Thanks for writing, db. Some climate scientists have noted the results from Christy et al in 2007 don't fully take into account uncertainties. A detailed analysis can be found in "Tropospheric temperature trends: history of an ongoing controversy" by Peter Thorne et al (Wiley Interdisciplinary Reviews: Climate Change, 2010). In it, the authors conclude: "The uncertainty of both models and observations is currently wide enough, and the agreement in trends close enough, to support a finding of no fundamental discrepancy between the observations and model estimates throughout the tropospheric column." Here's the link:
    • Quote 2
      db said on Nov. 19, 2010 at 8:05 a.m.
      I don't think trust is the issue because, climate models are projections (i.e. scenarios)and not predictions. I don't think anyone thinks that models will actually accurately 'predict' future climate conditions. Studies have shown that the IPCC models for instance can only 'predict' climate conditions out a year or two. The issue, as Dr. Judith Curry often points out, is that the uncertainty and inherent chaotic nature of climate systems is undervalued. As for the Santer et al., 2003 models of atmospheric temperature profiles, the actual data does not support the model. See Christy et al., 2007 Tropospheric temperature change since 1979 from tropical radiosonde and satellite measurements.
    • Quote 2
      Marc Airhart said on Nov. 17, 2010 at 10:47 a.m.
      Thanks for writing, jbw. These two graphs appeared in the IPCC's 4th Assessment Report in 2007 (Working Group 1 Report, Chapter 9) and are based on data from two peer-reviewed papers: Santer, B.D., et al., 2003a: Contributions of anthropogenic and natural forcing to recent tropopause height changes. Science, 301, 479–483. Stott, P.A., et al., 2006b: Transient climate simulations with the HadGEM1 model: causes of past warming and future climate change. J. Clim., 19, 2763–2782.
    • Quote 2
      jbw said on Nov. 15, 2010 at 6:58 p.m.
      Could you please provide full citations for the graphs.
    • Quote 2
      Marc Airhart said on Nov. 15, 2010 at 11:20 a.m.
      Thanks to the Know team for a speedy fix, the six-paneled graph with fingerprints of various climate forcings has now been updated to include labels. Go team!
    • Quote 2
      Marc Airhart said on Nov. 15, 2010 at 10:26 a.m.
      Thanks for writing, Nosmo. The unlabeled chart was my mistake. I've requested help replacing that with a labeled version. Check back in a few hours and be sure to clear your browser's cache to see that.
    • Quote 2
      Marc Airhart said on Nov. 15, 2010 at 9:07 a.m.
      Interglacial John, thanks for writing. We address the locations of weather stations and the urban heat island effect in Myth # 6. That's coming later today, so be sure to check it out. Once those issues are factored in, the climate record does show a significant global warming pattern during the 20th century, with accelerating warming from the 1970s onward.
    • Quote 2
      Charles Jackson said on Nov. 13, 2010 at 3:55 p.m.
      Despite the fact that human activity has increased atmospheric concentrations of sulfate aerosols which can cool climate, we observe planetary warming. This places some constraint on what aerosol cooling effects can be. If we find that the effects of aerosols is larger than expected, then our interpretation of the causes of the observed warming would require an increase in our interpretation of climate sensitivity to greenhouse gases. So what is stated above is that observations are consistent with the patterns of change we would expect from physics that we can represent reasonably well within climate models.
    • Quote 2
      Interglacial John said on Nov. 13, 2010 at 4:45 a.m.
      The reason the models show no warming when greenhouse gases are removed is because there has been no warming. If one studies properly sited weather stations that are not marine influenced one will ususally see either no warming or even a slight cooling over the past 100 years. Oceans heat and cool, cities have UHI, but the rural landlocked areas have not changed. AGW is a myth created to give power and money to those who wish to "change the world". My satisfaction comes in knowing that all the warmist predictions will fail. They know this too, that is why they have resorted to tampering with temperature records in a last ditch effort to convince the lemmings to jump. The Earth does not follow human direction and will out these frauds.
    • Quote 2
      Denis Ables said on Nov. 12, 2010 at 7:00 p.m.
      Here's some climate background for the un-initiated.
    • Quote 2
      Denis Ables said on Nov. 12, 2010 at 6:58 p.m.
      Computer models are NOT evidence of anything, except the modellers' interpretation of the system. If they tied their model to warming being caused by CO2, then no surprise that they get no warming when they remove the CO2 warming component.
    • Quote 2
      Nosmo said on Nov. 12, 2010 at 2:10 p.m.
      You are showing a graph with six panels but no explanation of what each is. How hard would it be to include that in the caption? Might as well not label the axes. (just a pet peeve)
    • Quote 2
      Marc Airhart said on Nov. 12, 2010 at 11:38 a.m.
      Martin, thanks for your response. In the 1990s, Roy Spencer and John Christy did publish papers suggesting the troposphere, or lower atmosphere, didn't show the predicted fingerprint of global warming. As later studies showed, and as John Christy acknowledges, there were errors in their original interpretation. The three independent teams tracking these trends (including the U. of Alabama) see a warming trend in the troposphere. An accessible description is at:
    • Quote 2
      Martin C said on Nov. 11, 2010 at 9:18 p.m.
      Please contact Dr. Roy Spencer at the University of Huntsville, Alabama, or Dr. Roger Pielke. Both have websites you can look up. Dr. Spencer researches tropospheric temperatures, and I am sure would have something to say about any supposed 'fingerprint' of the warming from greenhouse gases in the troposphere. Dr. Pielke has already made comments about unverifiable models on his website. It is clear this series is to convince the readers of 'man-made' global warming, and I suspect in later posts, will talk about how severe it is going to be. My issue is with the 'catastrophic' portion of global warming. I agree that the increase in CO2 likely has some small effect. I don't accept (based on a LOT of information I have read ever since the 2007 IPCC report came out) that it will ever be catastrophic, like James Hansen, Al Gore, and other say. I URGE anyone to look into this issue more themselves.
    • Quote 2
      Paintitblack said on Nov. 11, 2010 at 6:09 a.m.
      I'm on a forum where one poster makes the following claim:When the modellers predicted temperatures far higher than reality, they put their faith in 'global dimming', mostly from polluted clouds, nearly half present median AGW in AR4. If it’s not real, you must reduce predicted AGW by at least a factor of about 3. Investigating this kind of story is fun because you get into the heads of the people concerned. It starts with a Dutch academic, Van de Hout, who specialised in aerosols and liquid sols. He noticed there was more backscattered light than you'd intuitively expect. His approach was to 'lump parameters', a typical physicists' approach but fraught with problems. He linked the ‘Mie asymmetry factor’ with ‘optical depth’. The idea was adopted by cosmologist Carl Sagan to predict properties of Venusian clouds. Two associates, both now leading climate scientists, went to NASA where they used his equation for terrestrial clouds. It’s apparently in all the climate models and predicts pollution always increases cloud ‘albedo’, the desired ‘global dimming’. The problem is that by about 2004, experiment had proved there was no such effect except for thin clouds. Yet NASA claims on its websites enhanced ‘reflection’ from greater surface area of water in thick polluted clouds. There’s no such physics. The science bit is that Mie’s mathematics assumes a plane wave front. What pops out is scattered energy, strongly peaked in the forward direction, hence the asymmetry. Van de Hout went into his convoluted thinking because he assumed the same physics applied to subsequent interactions. Wrong: the mathematics changes dramatically. Until geometrical information is lost, lots more light is probably backscattered compared with the first interaction. The effect is far greater for larger droplets so pollution switches it off and much more light enters the cloud, another AGW. I wonder if you have any comments about these claims. Best regards, PIB
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