Climate Models and their Prediction of Climate Change

Climate model predictions

Keep in mind that there are many different climate models and that different models make different predictions about the future. The reason is that no model can fully represent all the complex process and feedbacks involved in the Earth system. Due to this uncertainty, we should consider all of them as possible outcomes of adding greenhouse gases.

The ability of global climate models to reproduce the observed surface temperature trends over the 20th century represents an important test of the models (see examples below). While most climate models are able to reproduce the slight warming in global average surface temperature that has been measured since 1860, no model is able to correctly get the spatial patterns of temperature changes correct.

Various modeling studies have suggested that a doubling of atmospheric carbon dioxide, or its equivalent by incorporating the effects of increases in other greenhouse gases, will rise mean global temperatures between 1.5 and 4.5°C. Climate scientists generally agree that if the Earth's atmosphere contained two times the amount of carbon dioxide as it did prior to the industrial revolution, then the Earth's average surface temperature would be 2.5-3.5°C warmer than pre-industrial times.

You must understand that these studies were done by doubling carbon dioxide, then letting the model run until a new equilibrium climate state was reached. In reality, the increases in greenhouse gases happen over an extended period of time and the climate system takes some time to come into equilibrium.

A big issue here is that the warming in surface temperatures tends to lag behind the increase in greenhouse gases. To a large degree, this lag is due to the large thermal inertia of the oceans -- in other words it takes a lot of energy to raise the temperature of the ocean water.

Let me try to break it down into understandable steps.

• Increasing the amount of greenhouse gases in the atmosphere, intially slows down the rate at which the Earth emits infrared radiation to space.
• Assuming no change in energy input (Energy the Earth absorbs from the Sun), the Earth is no longer in energy balance since energy in exceeds energy out.
• As long as energy in exceeds energy out, the planet warms. In particular the Earth's surface temperature will warm.
• As the Earth's surface temperature warms, the amount of radiation energy it emits increases. Since some of this extra radiation energy is lost to space, the amount of radiation energy that the planet emists to space increases.
• The warming continues until the energy out increases to the point where it equals the energy input.
• The Earth's surface is 70% covered by Oceans. Water has a high specific heat, so the Ocean surface temperature is slow to warm up in response to the energy imbalance caused by adding greenhouse gases to the atmosphere. In addition, the surface temperature of the ocean is slow to warm up because the additional heat must mix throughout the entire depth of the ocean.
• Therefore the amount of warming of the Earth's surface (temperature increase) required to bring the system back into balance (energy input equal energy output) will take quite some time.

As a result of the delay induced by the oceans, climate scientists do not expect the Earth to warm by the full 1.5-4.5°C (3-8°F) by 2060, even though the level of CO₂ is expected to have doubled by that time. We can make several conclusions here.

1. This delay provides an opportunity to reduce the magnitude of anthropogenic climate change before it is fully realized, if appropriate action could somehow be taken to reduce greenhouse gas concentrations.
2. Even if we were to stabalize greenhouse gas concentrations today, this delay means that the surface temperature will continue to warm until a new equilibrium is reached (this may take hundreds of years).
3. If we wait to act, still greater climate change will be expected, which will be more difficult or impossible to avoid.
4. The ocean surface temperature would also be slow to cool. Suppose in the future climate change gets out of control and we decide that we need to lower the average surface temperature of the Earth by somehow removing greenhouse gases from the atmosphere. The cooling would be very slow and lag behind the decrease in greenhouse gases. Thus global warming cannot be quickly reversed.

Currently, the Intergovernmental Panel on Climate Change (IPCC) projects a warming of 1.0-3.5°C (1.8-6.3°F) in global average temperature by the year 2100. This estimate is based on the latest runs of what are considered to be the best global climate models AND the most accepted estimates of future emissions of greenhouse gases. Keep in mind that we are never certain of future emissions of greenhouse gases, so often various emission senarios are run by modeling groups.

Directly below are a few figures that illustrate the global warming projections of one of the global climate models run by the Hadley Centre for Climate Prediction and Research (jointly funded by the United Kingdom Department of the Environment and the United Kingdom Meteorological Office).

Below is the projected from four different global climate models. Note the rather large differences between the various models in their prediction of the increase in global average surface temperature over the next 100 years or so. If you were to compare the regional changes in temperature predicted by each of these models, you would see even greater differences among them.

Climate Model Predictions for Extreme Weather Events

Recall when we started this section on global warming and climate change, it was pointed out that the frequency and intensity of extreme weather events is probably more influential on the types of plants and animals that can survive in a given ecosystem than the average conditions. Therefore, any changes in the distribution of extreme events is an extremely important thing to monitor and predict.

Predicting the distribution of extreme events is a very difficult problem for climate models to answer. For one, extremes are by definition rare, which makes statistical conclusions far more difficult to draw. Another reason is that the wildest weather is often confined to areas that are smaller than global climate models can predict (typical horizontal resolution of climate models is about 150 km). In a sense this is just a re-statement of the problem mentioned above: climate model projections are quite uncertain over small regional scales.

Because predicting and monitoring changes in the distribution of extreme weather events is so important to our understanding of the effects of global warming and climate change, research groups working with climate models are beginning to look at this issue. The following 10 indicies for extreme weather have been identified as target issues for climate prediction models:

• Total number of frost days
• Intra-annual extreme temperature range -- difference between the highest and lowest temperatures over a calendar year
• Growing season length -- begins after daily average temperature is above 41°F for 5 consecutive days; ends at start of 5 consecutive days when daily averge is below 41°F
• Heat wave duration index -- longest stretch of days with high temperatures at least 9°F warmer than the 1961-1990 average high temperature
• Warmth of daily lows -- Percentage of time daily low temperatures fall above the 90th percentile of daily minimum temperature for 1961-1990 average
• Wet days -- number of days with at least 0.40 inches of precipitation
• Dry days -- maximum number of consecutive days with no precipitation
• Wet stretches -- maximum 5-day precipitation total
• Average daily precipitation intensity -- Annual precipitation divided by the number of days in which precipitation fell
• Influence of very wet days on annual total -- Percentage of annual total precipitation occurring on days that exceed the 95th percentile from 1961-1990 average

Currently, we have low confidence in the ability of climate models to accurately predict the above indices. Hopefully, with lots of hard work, scientists can improve climate models to better answer the important question: how will the distribution and intensity of extreme weather events change in response to human activities?