Preliminary evidence shows that humans have been affected by recent increases in floods and droughts.

"Natural systems are vulnerable to climate change and some will be irreversibly damaged."

Among the most vulnerable natural systems are:

Greater climate change will mean greater loss of biodiversity - both in geographical range and intensity.

Some human systems are vulnerable

"Many human systems are sensitive to climate change, and some are vulnerable."

Impacts on agriculture are expected to be dominantly negative. Although in a few areas there may be minor increases in crop yields in the short term, as climate change deepens agriculture is expected to suffer almost everywhere.

Human settlements in low-lying regions are particularly vulnerable to flooding. The expected 21st century sea level rise, coupled with increased heavy precipitation events and strong storm surges, threatens hundreds of millions of people in densely populated areas (e.g. Bangladesh, Egypt's Nile Delta). This will cause loss of life and property as well as large scale migration.



Other vulnerable human systems include:

Extreme climatic events like droughts, floods and windstorms can be devastating to human societies. Most of these events are projected to increase in frequency and intensity during the 21st century. The economic and societal impacts of intensifying extremes are expected to fall disproportionately on the poor.

Catastrophic surprises are possible

The potential for large scale, irreversible impacts presents severe, but poorly understood risks.

Projected climate change could prompt sudden reorganizations in Earth systems with catastrophic ramifications. Unfortunately, our understanding of these possible impacts is too rudimentary to allow us to judge their likelihood. Because of this, they are not included in IPCC projections.

Plausible, but poorly understood possiblities include:

• Slowing of the ocean circulation that warms the North Atlantic. This would cause a sudden cooling of Northern Europe and drastically rearrange the global heat budget.
  
• Collapse of the Greenland or West Antarctic Ice Sheet. The rapid melting of either one of these ice sheets would cause 3m to 5m of global sea level rise.
  
• Accelerated greenhouse effects due to carbon cycle feedback. A snowball-effect could result if Earth systems react to warming by releasing more greenhouse gases from reservoirs such as permafrost.
  
• Massive release of methane from hydrates in coastal sediments. Hydrates contain vast amounts of methane gas in a relatively unstable form. Changes in ocean temperature or circulation could release this methane to the atmosphere, contributing to a "runaway greenhouse".