Climate Model Hindcasting

In this very readable article, Sawyer went over some of the evidence that humans are increasing the CO₂ concentrations by burning fossil fuels. He discussed what happens to CO₂ in the atmosphere and the ocean, and how this feeds back into other Earth system processes. Sawyer also acknowledged several of the factors which made (and still make) the warming effect harder to predict, and estimated how much impact those uncertainties could have on the prediction.

Sawyer stated that the CO₂ concentration was expected to increase by 25% by the year 2000. This implied Earth's temperature would increase by 0.6°C by the year 2000, assuming a climate sensitivity of 2.4°C.

This projection used a radiative energy balance model, which compared the rate at which Earth absorbs energy from the sun to the rate at which it is emitted into space. The difference was used to estimate the rate at which the Earth's temperature will rise. CO₂ and other greenhouse gases absorb some of the outgoing radiation and re-emit it in all directions, including back to Earth, reducing the amount of energy escaping into space. (In Sawyer's model, the only greenhouse gases included were CO₂ and water vapour.)

Sawyer, J.S. (1972). Man-made Carbon Dioxide and the "Greenhouse" Effect. Nature, 239, 23–26. https://doi.org/10.1038/239023a0

In addition to presenting a projection of the Earth's temperature in response to rising CO₂ concentrations, this article compared the CO₂ effects to known natural climate cycles observed in ice core temperature records. At the time of publication, the Earth's temperature had been cooling, which fit into the natural cycle pattern. Broecker's model predicted that the CO₂ effect would overwhelm the natural cycles within a decade or so.

This temperature prediction begain with estimates of the amount of fossil fuels that would be burned. CO₂ emissions were calculated from there, with some assumptions about what fraction of emitted CO₂ remains in the atmosphere. An energy balance model was then used to translate CO₂ levels into changes in global temperature.

Broecker noted that the main uncertainties in his model projections were the climate sensitivity parameter and the actual rate of CO₂ emission. He also observed that the effects of aerosols on global temperature were not yet well understood. However, Broecker emphasized that, in spite of these uncertainties, climate change due to increasing global temperatures was clearly coming.

This projection used a radiative energy balance model, which compared the rate at which Earth absorbs energy from the sun to the rate at which it is emitted into space. The difference was used to estimate the rate at which the Earth's temperature will rise. CO₂ and other greenhouse gases absorb some of the outgoing radiation and re-emit it in all directions, including back to Earth, reducing the amount of energy escaping into space. (In Broecker's model, the only greenhouse gas included was CO₂.)

Broecker, W. (1975). Climatic Change: Are We on the Brink of a Pronounced Global Warming? Science, 189, 460–463. https://doi.org/10.1126/science.189.4201.460

The Earth's temperature was not noticeably higher in 1981 than it was at the time of Broecker's model in 1975. Hansen et al. began their paper by noting this was "the major difficulty" in accepting the greenhouse effect and its consequences. They describe in detail which aspects of climate they were including in their model and how they were representing them. They also explored many of their assumptions, showing how their predictions could change if they made different reasonable choices.

Hansen et al. used hindcasting to show how well their model worked, as well as to constrain some of the important input values.

This model calculated temperature as a function of altitude (a "one-dimensional radiative-convective model"), which allowed the scientists to more accurately model greenhouse gas concentrations, clouds, and aerosols. This advance allowed them to account for how long it takes to heat up the ocean.

The paper cited estimates of climate sensitivity of 2–3.5°C.

Hansen, J., Johnson, D., Lacis, A., Lebedeff, S., Lee, P., Rind, D., Russel, G. (1981). Climate Impact of Increasing Atmospheric Carbon Dioxide. Science, 213, 957–966. https://doi.org/10.1126/science.213.4511.957

This was one of the first truly three-dimensional climate models, so it could track the interaction of land and ocean, simulate clouds, and more. This also meant it could predict how climate change would affect different parts of the world.

Understanding of the ocean was a major source of uncertainty for this model. Hansen et al. described their simplified ocean as a "surprise free" representation, and emphasized the need to understand it better to improve future models.

Hansen, J., Fung, I., Lacis, A., Rind, D., Lebedeff, S., Ruedy, R., Russell, G., and Stone, P. (1988). Global climate changes as forecast by Goddard Institute for Space Studies three-dimensional model. Journal of Geophysical Research, 93, 9341-9364, https://doi.org/10.1029/JD093iD08p09341

The First Assessment Report from the Intergovernmental Panel on Climate Change (IPCC) was released in 1990. The IPCC made the climate projections in this report in two (very large) stages: First, they used a variety of general circulation models from groups around the world to estimate the climate sensitivity and other parmaeters. They fed those results into a simpler energy balance model to make predictions about temperature and other aspects of climate change. The models used for the first stage were complex enough that it would have been almost impossible to explore a range of scenarios using them directly with the computer processing power available at the time.

IPCC (1990). Climate change: The IPCC scientific assessment. Report prepared by Working Group I. In J. T. Houghton, G. J. Jenkins, & J. J. Ephraums (Eds.), Intergovernmental Panel on Climate Change (p. 365). Cambridge, UK and New York, NY: Cambridge University Press. Available at the IPCC website.

The IPCC's projections in their Second Assessment Report used a similar procedure to their first report: detailed models from around the world were used to study the climate, and the IPCC combined their results using a simpler energy balance model to make its projections. Again, there was not enough computing power available to apply all of the models directly across a range of scenarios. The models included better treatment of anthropogenic aerosols, improved carbon cycle simulation, and more.

The scenarios in this report described emission of a variety of greenhouse gases, aerosols, and more, and they covered many different possible assumptions about government policies, demographic changes, and human behaviour. (We are representing them here as different CO₂ projections for simplicity.) The IPCC emphasized that these are "representative" scenarios to show what is likely to happen to the climate, and what can be done about it. There were many other possible assumptions and combinations that could be made. These would change the details of the projections, but would lead to the same conclusions: Our behaviour has serious consequences, but there was (and is) still time to change.

The different scenarios all lead to similar predictions for 2017; the differences between them are more obvious by mid-century or so.

Scenarios: IPCC (1992). In J.T. Houghton, B.A. Callander, S.K. Varney (Eds.), Climate Change 1992: The Supplementary Report to the IPCC Scientific Assessment. Cambridge, UK and New York, NY: Cambridge University Press. ISBN: 0-521-43829-2. Available at the IPCC website.

Report: IPCC (1996). In J. T. Houghton, L. G. Meira Filho, B. A. Callander, N. Harris, A. Kattenberg, & K. Maskell (Eds.), Climate change 1995: The science of climate change, Contribution of Working Group I to the Second Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK and New York, NY: Cambridge University Press. ISBN 0‐521‐56433‐6. Available at the IPCC website.

For the Third Assessment Report, the IPCC was able to use the results of almost 20 atmosphere-ocean GCMs from around the world, as well as other models for specific aspects of the climate. Many of these models included a wider range of greenhouse gases, improved input parameters, and more detailed studies of model uncertainties.

The IPCC's projections in their Third Assessment Report followed the same procedure as ealier: detailed models from around the world were used to study the climate, and the IPCC combined their results using a simpler energy balance model to make its projections. Again, there was not enough computing power available to apply all of the models directly across a range of scenarios. The time limit was even tighter this time, because the SRES scenarios had been only recently published when the IPCC wanted to produce their next Assessment Report.

The new "SRES" scenarios used in this report covered a wide range of possible global social and economic behaviours. All forty or so scenarios gave nearly the same predictions for the first couple of decades. Longer-term projections, on the other hand, depended much more strongly on the scenario.

Scenarios: IPCC (2000). Nebojsa Nakicenovic and Rob Swart (Eds.), Special Report on Emission Scenarios. Cambridge, UK: Cambridge University Press. Available at the IPCC website.

Report: IPCC (2001). In J. T. Houghton, Y. Ding, D. J. Griggs, M. Noguer, P. J. van der Linden, X. Dai, K. Maskell, & C. A. Johnson (Eds.), Climate change 2001: The scientific basis, Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK and New York, NY: Cambridge University Press. ISBN 0‐521‐80767‐0. Available at the IPCC website.

Major model improvements available for the IPCC's Fourth Assessment Report included more accurate and detailed atmospheric simulations and overall finer model resolution. In addition, a number of the models used had a combination of coarser and finer resolution. Important events such as cyclones, as well as Earth system processes such as vegetation growth and spread, often require very fine resolution to properly simulate and understand, but the computing power required to run the entire GCM at such fine resolution would be enormous. "Coupling" models at different scales will likely be an important technique in climate modeling well into the future.

Important Earth system processes were included in the models of the Fourth Assessment Report which had not been available earlier. Highlights include the biogeochemistry of carbon cycles, simulation of land ice dynamics, and atmospheric chemistry.

Scenarios: IPCC (2000). Nebojsa Nakicenovic and Rob Swart (Eds.), Special Report on Emission Scenarios. Cambridge, UK: Cambridge University Press. Available at the IPCC website.

Report: IPCC (2007). In S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor, & H. L. Miller (Eds.), Climate change 2007: The physical science basis, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK and New York, NY: Cambridge University Press. ISBN 978‐0‐521‐88009‐1. Available at the IPCC website.