Changing weather

During the 20th century we have witnessed a change in precipitation trends, temperature trends and increased sea levels. It is very likely that the 20th century warming has contributed significantly to the observed rise in global average sea level and increase in ocean-heat content. Increasing global mean surface temperature is very likely to lead to changes in precipitation. Extreme events are currently a major source of climate-related impacts. For example, heavy losses of human life, property damage, and other environmental damages were recorded during the El Niño event of the years 1997-1998.

Precipitation has very likely increased during the 20th century by 5 to 10% over most mid- and high latitudes of the Northern Hemisphere continents, but in contrast, rainfall has likely decreased by 3% on average over much of the subtropical land areas. There has likely been a 2 to 4% increase in the frequency of heavy precipitation events in the mid- and high latitudes of the Northern Hemisphere over the latter half of the 20th century. There were relatively small long-term increases over the 20th century in land areas experiencing severe drought or severe wetness, but in many regions these changes are dominated by inter-decadal and multidecadal climate variability with no significant trends evident over the 20th century.

Over the 20th century there has been a consistent, large-scale warming of both the land and ocean surface, with largest increases in temperature over the midand high latitudes of northern continents. The warming of land surface faster than ocean surface from the years 1976 to 2000 is consistent both with the observed changes in natural climate variations, such as the North Atlantic and Arctic Oscillations, and with the modelled pattern of greenhouse gas warming.

It is very likely that the 20th century warming has contributed significantly to the observed rise in global average sea level. Warming drives sea-level rise through thermal expansion of seawater and widespread loss of land ice. Based on tide gauge records, after correcting for land movements, the average annual rise was between 1 and 2 mm during the 20th century. The very few long records show that it was less during the 19th century. The observed rate of sea-level rise during the 20th century is consistent with models. Global ocean-heat content has increased since the late 1950s, the period with adequate observations of subsurface ocean temperatures.

A limited number of sites in Europe have nearly continuous records of sea level spanning 300 years and show the greatest rise in sea level over the 20th century. Records shown from Amsterdam, The Netherlands, Brest, France, and Swinoujscie, Poland, as well as other sites, confirm the accelerated rise in sea level over the 20th century as compared to the 19th.

Extreme weather

The number of weather-related catastrophic events has risen three times faster than the number of non-weatherrelated events, despite generally enhanced disaster preparedness.

The economic losses from catastrophic weather events have risen globally tenfold (inflation adjusted) from the 1950s to the 1990s, much faster than can be accounted for with simple inflation. The insured portion of these losses rose from a negligible level to about 23% in the 1990s. The total losses from small, non-catastrophic weather-related events (not included here) are similar. Part of this observed upward trend in weather-related disaster losses over the past 50 years is linked to socio-economic factors (e.g., population growth, increased wealth, urbanisation in vulnerable areas), and part is linked to regional climatic factors (e.g., changes in precipitation, flooding events).




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