Global Climate Change

What we know, what we are fairly confident about, and where

the uncertainties lie.

What do we know?

• CO2, methane, CFC’s, water vapor trap heat (= “greenhouse gases”).

• Global average temperature has risen 0.3-0.6 C in the last 100 years.

• Global average sea levels rose 10-25 cm over the last 100 years.

• The global climate system is very complex.

What do we confidently conclude?

• The rise in CO2 is the largely the result of human activity.

• More CO2 will probably mean further rise in global average temperatures and sea levels.

• Sulfate aerosols have been counteracting the warming trend.

What do we not really know?

• Regional changes are very hard to predict at present. Local conditions may change much more than the global average.

• Whether regional weather patterns are becoming more extreme or variable.

Major sources of uncertainty

• Clouds: warming may mean more clouds, but these could cool or warm the planet.

• Oceans: the fate of CO2 over the long term in the ocean is not known.

• Interactive effects. Poorly known links in the climate system may lead to surprising changes.