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CGD Climate Highlights: Climate Change

Why do we believe the climate is changing?

Modelling and Attribution of Observed Climate Change

The best climate models encapsulate the current understanding of the physical processes involved in the climate system, the interactions, and the performance of the system as a whole. They have been extensively tested and evaluated using observations. They are exceedingly useful tools for carrying out numerical climate experiments, but they are not perfect, and some models are better than others. Uncertainties arise from shortcomings in our understanding of climate processes operating in the atmosphere, ocean, land and cryosphere, and how to best represent those processes in models. Yet, in spite of these uncertainties, today's best climate models are now able to reproduce the climate of the past century, and simulations of the evolution of global surface temperature over the past millennium are consistent with paleoclimate reconstructions.

As a result, climate modelers are able to test the role of various forcings in producing the observed changes in global temperature temperatures. Forcings imposed on the climate system can be natural in origin, such as changes in solar luminosity or volcanic eruptions, the latter adding considerable amounts of aerosol to the upper atmosphere for up to two years. Human activities also increase aerosol concentrations in the atmosphere, mainly through the injection of sulfur dioxide from power stations and through biomass burning. A direct effect of sulfate aerosols is the reflection of a fraction of solar radiation back to space, which tends to cool the Earth's surface. Other aerosols (like soot) directly absorb solar radiation leading to local heating of the atmosphere, and some absorb and emit infrared radiation. A further influence of aerosols is that many act as nuclei on which cloud droplets condense, affecting the number and size of droplets in a cloud and hence altering the reflection and the absorption of solar radiation by the cloud. The precise nature of aerosol/cloud interactions and how they interact with the water cycle remains a major uncertainty in our understanding of climate processes. Because man-made aerosols are mostly introduced near the Earth's surface, they can be washed out of the atmosphere by rain. They therefore typically remain in the atmosphere for only a few days, and they tend to be concentrated near their sources such as industrial regions. Therefore, they affect climate with a very strong regional pattern and usually produce cooling.

In contrast, greenhouse gases such as carbon dioxide and methane are not washed out, so they have lifetimes of decades or longer. As a result, they build up in amounts over time, as has been observed. Greenhouse gas concentrations in the atmosphere are now higher than at any time in at least the last 750,000 years. It took at least 10,000 years from the end of the last ice age for levels of carbon dioxide to increase 100 parts per million by volume (ppmv) to 280 ppmv, but that same increase has occurred over only the past 150 years to current values of over 370 ppmv. About half of that increase has occurred over the last 35 years, owing mainly to combustion of fossil fuels and deforestation. In the absence of controls, future projections are that the rate of increase in carbon dioxide amount may accelerate, and concentrations could double from pre-industrial values within the next 50 to 100 years.

Climate model simulations that account for such changes in forcings have now reliably shown that global surface warming of recent decades is a response to the increased concentrations of greenhouse gases and sulfate aerosols in the atmosphere. When the models are run without these forcing changes, they fail to capture the almost linear increase in global surface temperatures since the mid-1970s. But when the anthropogenic forcings are included, the models simulate the observed temperature record with impressive fidelity. These same model experiments also reveal that changes in solar luminosity account for much of the warming in the first half of the 20th Century. Such results increase our confidence in the observational record and our understanding of how temperature has changed. They also mean that the time histories of the important forcings are reasonably known, and that the processes being simulated models are adequate enough to make the models very valuable tools.