Introduction to the Science of Climate Change
Prof. Jordi Miralda-Escudé, F 9:30
Lecture 8: Climate Models for the Future.
Scientists have defined a certain number of scenarios on what future climate change may be like. One should be careful not to confuse scenarios with predictions. A scenario proposes certain assumptions about how future emissions of greenhouse gases will increase, and then it shows how the global temperature would increase if these emissions increased as assumed. Of course, the actual emissions in the future will depend on many factors, such as population growth, economic growth, technological advances, and political action to reduce greenhouse emissions. A scenario for these future emissions does not imply that a prediction is being made that these emissions will be as assumed. The scenarios are a tool for the public and politicians to understand what the likely consequences for the climate are depending on the future emissions, so that we can then decide on what actions to take to reduce our emissions. It is important to understand that the temperature increase expected under each scenario is subject to a number of uncertainties.
Once a possible scenario has been defined for future emissions of carbon dioxide and other greenhouse gases, and other climate agents like sulfate aerosols and black carbon, there are a number of uncertainties that affect the final outcome for climate change:
What will be the future concentration of greenhouse gases?
For fixed future emissions of carbon dioxide under a specific scenario, the future concentration in the atmosphere will depend on how much carbon dioxide will be absorbed by the land and the ocean. Under increasing carbon dioxide in the atmosphere, it is expected that vegetation, soils, and the ocean will continue to increase their annual uptake of carbon dioxide. However, the rate of increase of this uptake may be reduced by the warming of the planet itself. There are uncertainties on how much additional carbon soils and plants will be able to retain in a warmer world with more carbon dioxide in the atmosphere.
The ocean also diminishes its ability to absorb carbon dioxide as it warms up. The biological cycles that depend on circulation of nutrients from the deep to the surface ocean and alter the rate of exchange of carbon dioxide with the atmosphere may be affected by ocean warming.
How much warmer will the Earth become?
After a model has specified the way in which the greenhouse gases and aerosols will change their concentrations, there are still other uncertainties related to climate sensitivity , or how much the temperature will change under a fixed radiative forcing. As described in Lecture 7, this climate sensitivity depends on feedback effects: as the planet warms up, increased water vapor and changes in clouds can enhance the warming by an amount that is uncertain.
The Intergovernmental Panel on Climate Change has presented results for the increase in temperature over the 21st century based on several scenarios. The average temperature of the Earth is expected to increase by an amount between 3 and 10 degrees Fahrenheit based on these scenarios.
Possible threats from climate change.
Global warming over the 21st century may at first sight seem like a small change in climate we can easily adapt to: if we limit the temperature increase to, say, five degrees Fahrenheit, that is still less than the natural variability in temperatures we experience from one year to the next at any one location on Earth due to random weather variations. However, many aspects of Earth's climate are affected by an increase of the average temperature over all the planet and all the time, which could result in a dangerous outcome for human civilization:
Sea-level rise due to ice melting is much more uncertain. The rate at which glaciers will melt as the planet warms depends on many uncertain factors, such as disintegration of ice sheets when meltwater or rainwater flows down to the base of the glacier and lubricates the base, helping the glacier to slide down. All the ice in Greenland is enough to raise sea-level by 7 meters, and the ice in Antarctica is enough to raise sea-level by 50 meters. No one believes all the ice could possibly melt in the 21st century, but only a small fraction would already cause devastating sea-level rise. About 300 million people in the world live in areas that would become inundated by a sea-level rise of only 1 meter.
Sea-level rise is a greater threat when we think beyond the 21st century. There is long delay between the warming of the planet and the resulting sea-level rise, because it takes a long time for the ocean to warm up and expand and for the glaciers to melt. Once the sea has warmed and the ice has melted, it also takes a long time (thousands of years) after greenhouse gases decrease in the future for ice to form again and the sea-level to drop.
Although the fact that the mean temperature and the mean rate of precipitation will increase during the 21st century as a result of the anthropogenic greenhouse gases is a very solid prediction, the other consequences of this warming related to the threats discussed above are very uncertain. We can say that there will be a warming and that this entails certain risks, although we cannot be sure if any of these risks will actually materialize.