Modeling Weather and Climate

Climate Models and their Prediction of Climate Change

The Greenhouse Effect (A Review)

There is a natural greenhouse effect which keeps the Earth warm enough (average temperature 60oF) to be habitable. Greenhouse gases like carbon dioxide, methane, nitrous oxide and water vapor enhanced the infrared radiation emitted by the atmosphere and warm the earth's surface. The basic principles of the greenhouse effect and its enhancement are well understood. For a given concentration of greenhouse gases, the resulting amount of radiative forcing can be predicted with precision. Exactly how the Earth's climate will respond to enhanced greenhouse gases will also depend on complex interactions between the atmosphere, oceans, land, ice, and biosphere.

Atmospheric concentrations of greenhouse gases have increased significantly since industrial revolution: Carbon dioxide +33%; Methane +157%; Nitrous oxide +17%.

Many greenhouse gases remain in atmosphere for a long time (decades to centuries).

A rise in all greenhouse gases, equivalent to a doubling of the pre-industrial level of carbon dioxide, is likely to be reached by the middle of the next century (by about 2060).

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, may rise mean global temperatures between 1.5 and 4.5°C, with a best guess at around 2.5-3.5°C.

Since 1979, scientists have generally agreed that a doubling of atmospheric carbon dioxide increases the earth's average surface temperature by 1.5-4.5°C (3-8°F). 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.

As an example, the warming in surface temperatures tends to lag behind the increase in greenhouse gases. At first, the cooler oceans will tend to absorb much of the additional heat and thereby slow down the warming of the atmosphere. This is due to the high specific heat of liquid water, therefore, the oceans are slow to warm, but once warmed they will also be slow to cool. Only when the ocean comes into equilibrium with the higher level of CO₂ will the full warming occur.

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.

Currently, the Intergovernmental Panel on Climate Change (IPCC) projects a warming of 1.0-3.5°C (1.8-6.3°F) by the year 2100. This estimate is based on the latest runs of what are considered to be the best global climate models.

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).

Finally, here 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.