An illustration of this slowness can be the reduction of glaciers that has been measured in certain areas; although they have been reduced by about one-third over the last 135 years, this is mainly because the Earth is still returning to its normal state after a temporary cooling during the middle ages (the "Little Ice Age" from 1400 to 1800)!
Because the climate system is so slow, the climate will continue to warm up even after emissions are stabilized or reduced. The sea level will continue to rise for many hundreds of years after CO2 emissions are stabilized.
The slowness also adds to the uncertainties in our knowledge about climate changes. Both the degree to which people add to the greenhouse effect and the possible consequences of human actions are difficult to study because it takes so long from when a gas is emitted to when we can measure the changes. And when scientists try to estimate how the future will be, it takes a long time from when they make the calculations to when they can check to see if they were right.
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SLOW RESPONSE: This figure shows what might happen if man-made emissions of CO2 stop growing at some point during the next hundred years, and then start falling. After CO2 emissions are reduced and concentrations in the atmosphere stabilize, surface air temperature continues to rise slowly for a century or more. Expansion of the ocean due to of warmer water continues long after CO2 emissions have been reduced, and melting of ice sheets continues to contribute to sea-level rise for many centuries. Source: IPCC (click to enlarge, 48 kB)
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