Scientists' Explanation for Investigation 1

Scientists project global mean temperature to increase by approximately 1°C (1.8°F) for the lowest 2.6 emissions scenario and 4°C (7.2°F) for the highest 8.5 emissions scenario by 2100. This projected global mean temperature change for the 21st century will depend on which scenario actually occurs. The low emissions scenario assumes that humans will aggressively reduce emissions so that the increase by 2100 is only 2°C (3.6°F) higher than pre-industrial levels. This 2°C seems to be a critical number for many scientists, including the IPCC.

This change in temperature may seem insignificant to you; however, these are global averages and not daily variations. The world will be an entirely different place with a 2°C difference. For some areas, the difference may actually be beneficial but for others it will be devastating. The higher polar latitudes will be even warmer due to the positive feedback in the climate system. (That is, as the Arctic sea ice melts and albedo or reflectivity decreases, even more heat will be absorbed, causing amplified warming.) And consider the opposite––the last time Earth was 2°C colder than pre-industrial levels. This occurred approximately 18,000 years ago during a glacial period when the Laurentide ice sheet covered vast areas of North America.

Remember, an increase of 2°C (3.6°F) by 2100 is for the low emissions scenario. Continuing on the “business as usual” path that we are on now, we are heading toward the higher emissions scenario, resulting in temperatures that are 5°C (9°F) above pre-industrial levels. The world will be a far different place than it is today with many of the same impacts that we are seeing today but occurring at an accelerated rate. You will learn more about these impacts in the next investigations.

The measure of how much mean global surface temperature will change as energy is added to or subtracted from the climate systems is known the climate sensitivity. Climate scientists use a combination of paleoclimate data, observations, and computer models to estimate how sensitive the climate is to a particular forcing (such as atmospheric greenhouse gas concentration).

Climate sensitivity is most commonly expressed at the increase in temperature expected after doubling the preindustrial concentration of CO₂ from 280 ppm to 560 ppm. The basic physics indicates that a doubling will increase global temperature approximately 1°C (1.8°F). However, positive and negative feedbacks (such as the melting of the Arctic sea ice or the reflection of clouds) within the climate system can affect the sensitivity. Global temperature has already increased by 1°C (1.8°F) and the IPCC estimates climate sensitivity between 1.5°C (2.7°F) and 4.5°C (8.1°F). This means that with a doubling of CO₂, in a few hundred years (once the climate reached a new equilibrium), the global mean temperature would be between 1.5°C (2.7°F) and 4.5°C (8.1°F).