A climate forcing is an imposed perturbation of the Earth's energy balance. If the sun brightens, that is a positive forcing that warms the Earth. Aerosols (fine particles) blasted by a volcano into the upper atmosphere reflect sunlight to space, causing a negative forcing that cools the Earth's surface. These are natural forcings. Human-made gases and aerosols are also important forcings.
Climate sensitivity is the response to a specified forcing, after climate has had time to reach a new equilibrium, including effects of fast feedbacks. A common measure of climate sensitivity is the global warming caused by a doubling in atmospheric CO2 concentration. Climate models suggest that doubled CO2 would cause 3 °C global warming, with an uncertainty of at least 50%. Doubled CO2 is a forcing of about 4 W/m2, implying that global climate sensitivity is about 3/4 °C per W/m2 of forcing.
Climate response time is the time needed to achieve most of the climate response to an imposed forcing, including the effects of fast feedbacks. The response time of the Earth's climate is long, at least several decades, because of the thermal inertia of the ocean and the rapid mixing of waters within the upper few hundred meters of the ocean. Climate sensitivity and response time depend upon climate feedbacks, which are changes in the planetary energy balance induced by the climate change that can magnify or diminish climate response. Feedbacks do not occur immediately in response to a climate forcing; rather, they develop as the climate changes.
Fast feedbacks come into play quickly as temperature changes. For example, the air holds more water vapor as temperature rises, which is a positive feedback magnifying the climate response, because water vapor is a greenhouse gas. Other fast feedbacks include changes of clouds, snow cover, and sea ice. It is uncertain whether the cloud feedback is positive or negative, because clouds can increase or decrease in response to climate change. Snow and ice are positive feedbacks because, as they melt, the darker ocean and land absorb more sunlight.
Slow feedbacks, such as ice sheet growth and decay, amplify millennial climate changes. Ice sheet changes can be treated as forcings in evaluating climate sensitivity on time scales of decades to centuries.
Climate sensitivity is the response to a specified forcing, after climate has had time to reach a new equilibrium, including effects of fast feedbacks. A common measure of climate sensitivity is the global warming caused by a doubling in atmospheric CO2 concentration. Climate models suggest that doubled CO2 would cause 3 °C global warming, with an uncertainty of at least 50%. Doubled CO2 is a forcing of about 4 W/m2, implying that global climate sensitivity is about 3/4 °C per W/m2 of forcing.
Climate response time is the time needed to achieve most of the climate response to an imposed forcing, including the effects of fast feedbacks. The response time of the Earth's climate is long, at least several decades, because of the thermal inertia of the ocean and the rapid mixing of waters within the upper few hundred meters of the ocean. Climate sensitivity and response time depend upon climate feedbacks, which are changes in the planetary energy balance induced by the climate change that can magnify or diminish climate response. Feedbacks do not occur immediately in response to a climate forcing; rather, they develop as the climate changes.
Fast feedbacks come into play quickly as temperature changes. For example, the air holds more water vapor as temperature rises, which is a positive feedback magnifying the climate response, because water vapor is a greenhouse gas. Other fast feedbacks include changes of clouds, snow cover, and sea ice. It is uncertain whether the cloud feedback is positive or negative, because clouds can increase or decrease in response to climate change. Snow and ice are positive feedbacks because, as they melt, the darker ocean and land absorb more sunlight.
Slow feedbacks, such as ice sheet growth and decay, amplify millennial climate changes. Ice sheet changes can be treated as forcings in evaluating climate sensitivity on time scales of decades to centuries.
