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Section 12.2 Anthropogenic climate change

There are a number of key indicators (e.g., see the NASA Vital Signs site: that point to a warming planet:

  • increased land and sea temperatures

  • lowered sea ice and land ice coverage

  • higher sea levels

It is important to note land temperatures have generally increased more than ocean temperatures because of differences in their thermal properties; so global average temperature must consider both contributions.

What is the difference between global warming and climate change?

The vital signs noted above are all consequences of increasing global temperatures - a phenomenon known as global warming. Higher average global temperatures can influence the climate in a number of ways; such phenomena referred to more broadly as climate change.

For example, a decrease in the latitudinal temperature gradient (a consequence of a warming planet) can result a jet stream more prone to stuck meridional patterns. This can variously give drought or flood conditions that are not simply attributed to a warming trend itself.

Why are *humans* thought to be causing this change?

Much of this change appears to be driven by observed increases in key greenhouse gas concentrations, such as:

  • carbon dioxide (from 280 ppm in 1880 to 420 ppm in 2021)

  • methane (from 500 ppb to 1900 ppb)

  • nitrous oxide (from 270 ppb to 330 ppb)

so that a larger proportion of solar radiation is effectively absorbed in the Earth's lower atmosphere. These increases in concentration are clearly attributed to anthropogenic emissions.

Since *water* is the most important greenhouse gas, why don't we worry about that instead of carbon dioxide?

  • water is most important greenhouse gas but at large scales it is not directly influenced by human activities

  • water has a very short residence time and water is subject phase changes at Earth's surface. In other words, the temperature controls the amount of water vapor that can be present in the atmosphere (and not vice versa). If this temperature-dependent limit is exceeded (the dew point) water will condense.

  • note that the amount of water vapor in the atmosphere is expected to slightly increase with increasing global temperatures: thus, water vapor feedback may serve as a positive feedback that amplifies warming

  • however, increasing water vapor in the atmosphere could also be expected to promote cloud formation, which would serve as a negative feedback that mitigates warming