To get a better idea of the amount of warming that can be expected, scientists use computer models to simulate our world with increased amounts of greenhouse gases in the atmosphere. Computer projections are used because current levels of CO2 at 353 ppm are much higher than the historical levels of CO2 recorded in arctic ice packs. (Historical fluctuations have ranged from 180 ppm during glacial times to 280 ppm for the warmer interglacial periods.)
The most sophisticated computer programs are called GCMs, or Global Circulation Models, and they are run on powerful supercomputers capable of many million floating-point calculations per second. The model is constructed by dividing the earth's surface up in a grid-like pattern, sampling perhaps 1000 to 20,000 evenly spaced points around the globe. The models are three-dimensional as well, for at each surface point location, the solar radiation, temperature, gas concentrations, air pressures, wind velocities and many other parameters are maintained for as many as 10 or 15 different elevation levels. Using this 3-dimensional data structure, scientists then program the model to calculate the interaction of the variables mentioned above according to the various laws of physics governing energy and mass transfer: refection, absorption, convection, conduction, and many others. The total number of data elements and computations are staggering - a single GCM run can take at least 100 hours of CPU time on a supercomputer.
Future climate modeling research using GCM's is currently taking place at 4 or 5 major labs, including NASA's Goddard Institute for Space Studies, NOAA's Geophysical Fluid Dynamics Lab, and NOAA's National Center for Atmospheric Research. To compare results between the different models, scientists have developed a standard experiment that asks the question - "how much will the average global temperature increase if the CO2 concentration in the air is doubled to 660 ppm?" Most current models show increases of between 6.3 and 9 degrees F (3.5 to 5 deg. C). In addition, some model runs will generate additional results, such as maps of expected rainfall levels over the continents, or moisture levels in the soil. Furthermore, these computer model simulations are run long enough into the future until results reach an equilibrium. Equilibrium occurs when the earth's temperature has finally stabilized at some higher temperature to accommodate the new CO2 'doubled' atmosphere.
The 660 ppm doubling figure may seem like a very high one, since CO2 levels are now at 353 ppm. However, these models use CO2 as the only greenhouse gas, and the effects of the other gases must be included to calculate the correct warming due to all the greenhouse gases. Assuming a continuation of present emission rates, V. Ramanathan from the University of Chicago estimates that the equivalent CO2 concentration could reach the 660 level within 45 years, by 2035. If only the CO2 rate were used, the concentration doubling would not occur until the year 2085.[Senate Comm: Energy and Nat. Resources, 11/9/87]
There is considerable public debate about the supposed accuracy or inaccuracies of these climate-CO2 models. Some argue that no action should be taken until more precise estimates and effects of warming can be made, while others argue strong actions to limit the production of greenhouse gases should be taken immediately before precious time to react is lost. Most scientists directly involved with this research feel that society needs to start acting to reduce global warming now. All the models predict climate change rates much greater than the planet has ever experienced before. Another reason for not waiting is that it will take 10 to 20 more years to improve the models and computer technology to the point where very accurate predictions are possible.
Let's continue our exploration of the science of climate modeling by first looking at results generated by the current models. Finally, we will see where the models are weak, and what improvements are needed in future research efforts.
Most importantly, all the computer simulations show significant temperature increases for the levels of greenhouse gases anticipated within the next 40 years. Models also report that warming at the high latitudes and middle latitudes will be more severe than the average warming, while equatorial areas will heat up less than average. Most models predict less rainfall in the high to mid latitudes of both North American and Europe- Asian continents, with the India-Bangladesh area the only one receiving increased amounts of moisture. In one comparison of 5 different models, all five predicted that the western midwest area around Omaha, Nebraska would receive less moisture under the doubled CO2 scenario.[Global Warming,Schneider,Sierra Books,1989]
The world's oceans probably hold the biggest uncertainty in climate prediction. First, there is the problem of determining how much heat storage capacity the oceans have - and they have a lot, since a certain mass of water can store more heat than the equivalent mass of most other common materials. In fact, at the present time, even the basic 'time constant', or time delay factor of the oceans is not well known. It is believed to be something greater than 20 years and yet less than 100 years. The oceans therefore represent a large inertia. Anything possessing high inertia is both hard to start moving and hard to stop as well. For decades, oceans absorbed the excess heat from the greenhouse effect, making detection of the 'greenhouse warming signal' difficult until now. But now global temperatures are on the move and it will be hard to control them. This is why researchers say that even if we, the citizens of the planet, were to somehow decide to control all greenhouse gases and keep them at today's levels, the earth will probably heat up another 1.8 degrees F (1.0 deg.C) by the year 2040.
A more subtle problem is understanding and modeling the exact flow of the ocean's currents. Because of the powerful flow of ocean currents, large amounts of heat can be redistributed around the globe in ways that are not yet understood. The direction, depth and strength of these currents are controlled by such factors as the amount of salt in the water, the temperature differences in various parts of the ocean basins, and the shape and depth of the ocean floor. Good measurement data on ocean currents, measured on a worldwide basis, simply are not available at this time. Stephen Schneider, a principal climatologist at the NOAA's Center for Atmospheric Research says that future models must integrate the three-dimensional modeling of both the atmosphere and the oceans before truly accurate results can be obtained. The lack of good ocean circulation data in today's models is probably the reason why the models seem to give reasonable weather estimates for large general areas of the globe, but often give conflicting predictions for a regions smaller that a continent, especially in coastal areas.
A number of other factors that affect global warming are not included in today's computer models. Various feedback mechanisms, particularly those involving biological processes, have yet to be included. Some are 'negative feedbacks', which should slow the temperature rise, while 'positive feedbacks' will accelerate the temperature changes. An example of a negative feedback mechanism would be that trees may absorb more CO2 when they encounter the higher CO2 concentrations in the future. An example of a positive feedback is when the permafrost tundra soils are heated, the thawing of the soils will release methane locked in icy structures called clathrates. Higher temperatures are also known to accelerate the decay of organic matter by bacteria. A 1 degree C rise can potentially generate a 30% increase in the release of methane from soils. The response of the ocean's phytoplankton to increased levels of CO2 and ultraviolet radiation (resulting from the loss of ozone from CFCs) is also not understood.
There are many areas where models need improvement. Wallace Broecker, scientist at Columbia University, gave this list of needed improvements at his testimony before a Senate hearing in 1987:
1. The large scale circulation of the ocean.
2. The processes regulating soil moisture.
3. The processes responsible for cloud formation.
4. The influence of the global biogeochemical cycles on the atmosphere's composition.
5. The processes regulating sea ice.[Jt.Hearing - Subcomm: Env. Protection and Hazardous Wastes, Jan 28, 1987] ***
