Climate FAQs: Global Temperatures
What has caused global temperatures to vary naturally by 5 to 7 degrees thousands of years ago before humankind started burning fossil fuels and releasing large amounts of carbon dioxide and other greenhouse gases?
Climate varies naturally. We consider natural variability as resulting from purely internal atmospheric processes as well as from interactions among the different components of the climate system, such as those between the atmosphere and oceans or the atmosphere and land. However, the most significant forcings with impact on climatic time scales are generally imposed upon the climate system.
External forcings arise from a wide array of processes covering a range of spatial and temporal scales. "Natural" external forcings include changes in the global configuration of the continents, the slow increase of solar luminosity that occur over hundreds of millions of years, variations in the Earth's orbit, and the injection of aerosols high into the atmosphere by explosive volcanic eruptions. Human emissions of carbon dioxide and other greenhouse gases, the local emission and suspension of small (aerosol) particles on timescales of minutes to days, and changes in land use are some examples of anthropogenic forcings.
The global temperature variations reflected in ice core records from the distant past reflect the influence of natural external forcings on the climate system. However, these reconstructions of past temperature swings have also demonstrated that the projected rate of global temperature change exceeds anything seen in nature in the past 10,000 years.
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 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.