Higher global temperatures are projected to have a wide range of impacts with potentially severe consequences for human societies and natural ecosystems, including rising sea levels, increased rates of infectious disease, heat waves, droughts, more frequent and severe wildfires, floods, extreme weather events, melting polar and glacier ice, species extinctions, shifts in species distributions, changing seasonal patterns, etc.
Many of these anticipated changes are already occurring. The 1990s were the warmest decade on record. The five warmest years since 1860, in decreasing order were: 1998, 2002, 2001, 1995, and 1997. Currently, this year (2003) is on track to become the third warmest year ever recorded.
Thus, the past three years will account for three of the four warmest years on record. Sea levels have risen 4 to 10 inches over the past century and are projected to rise an additional 3.5 to 34.6 inches by 2100, due primarily to thermal expansion, as well as melting glaciers and ice caps, with potentially devastating consequences for small island nations, coastal cities and wetland ecosystems around the world.
The Arctic ice cap has decreased in area by 34,300 km (the approximate size of the Netherlands) each year since 1978 (and has thinned over 40% in the past 35 years.
The geographical ranges of infectious disease vectors that transmit malaria, dengue fever, and encephalitis are expanding (e.g. mosquitoes, which are highly sensitive to changes in temperature and precipitation). Worldwide economic losses due to weather disasters have increased from under $4 billion in the 1950s to over $39 billion in the 1990s, adjusted for GDP. The projected impacts of relatively linear, incrementally warmer global temperatures on human societies and natural ecosystems are already dramatic.
The possibility of large, abrupt swings in regional and global climate, however, are potentially even more serious. Confronted with these and many other research findings, many scientists have called for immediate, concerted action to reduce global emissions of greenhouse gases.