Satellites represent a vital observing platform to monitor the climatic environment of global ecosystems. Importantly, they provide time series information that are essential to understand the dynamics governing changes in ecosystems due to multiple stresses imposed by human activities and natural causes. Understanding the external forces that drive changes in ecosystems also helps in the understanding of how ecosystems might change in the future, due, for example, to global climate warming as projected by the international Intergovernmental Panel on Climate Change (IPCC; see http://www.ipcc.ch). The most recent assessment (Fourth Assessment Report, often abbreviated as IPCC-AR4) on the science and impacts of global climate change has just been released. The reader is referred to the IPCC web site maintained by the UK Hadley Centre for various summaries and the status of the release of the findings of the Working Groups of the IPCC: http://www.metoffice.go-v.uk/research/hadleycentre/ar4/index.html.
The analysis of observed changes in the climate system reported by the IPCC are based on a combination of data from surface-based instrument networks, in situ observations, and a large number of operational and research satellite monitoring platforms. Selected findings of the IPCC excerpted from the summary for policymakers of IPCC Working Group I include the following:
• Eleven of the last 12 years (1995-2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850).. . .The linear warming trend over the last 50 years [0.13 °C (0.1-0.16 °C) per decade] is nearly twice that for the last 100 years.
• New analysis of balloon-borne and satellite measurements of the lower- and mid-troposphere temperature show warming rates that are similar to those of the surface temperature record.
• The average atmospheric water vapor content has increased since at least the 1980s over land and ocean as well as in the upper troposphere.
• Observations since 1960 show that the average temperature of the global ocean has increased to depths of at least 3000 m and that the ocean has been absorbing more than 80% of the heat added to the climate system. Such warming causes seawater to expand, contributing to sea level rise.
• Mountain glaciers and snow cover have declined on average in both hemispheres. Widespread decreases in glaciers and ice caps have contributed to sea level rise (ice caps do not include contributions from the Greenland and Antarctic Ice Sheets). New data (since the TAR (The Assessment Report of the IPCC)) now show that losses from the ice sheets of Greenland and the Antarctic have very likely contributed to sea level rise over 1993-2003. Flow speed has increased for some Greenland and Antarctic outlet glaciers, which drain ice from the interior of the ice sheets.
• Average Arctic temperatures increased almost twice the global average rate in the past 100 years. Satellite data since 1978 show that annual average Arctic sea ice extent has shrunk by 2.7% [2.1-3.3%] per decade with a larger increase in summer of 7.4% [5.0-9.8%].
• Temperatures at the top of the permafrost layer have generally increased since the 1980s in the Arctic (by up to 3.0 °C). The maximum area covered by seasonally frozen ground has decreased by about 7% in the Northern Hemisphere since 1900, with a decrease in spring of up to 15%.
• Long-term trends from 1900 to 2005 have been observed in precipitation amount over many large regions. Significantly increased precipitation has been observed in eastern parts of North and South America, Northern Europe, and northern and Central Asia. Drying has been observed in the Sahel, the Mediterranean, southern Africa, and parts of southern Asia. Precipitation is highly variable spatially and temporally, and data are limited in some regions. Long-term trends have not been observed for the other large regions assessed.
• Changes in precipitation and evaporation over the oceans are suggested by freshening of mid- and high-latitude waters together with increased salinity in low-latitude waters.
• Mid-latitude westerly winds have strengthened in both hemispheres since the 1960s. More intense and longer droughts have been observed over wider areas since the 1970s, particularly in the tropics and subtropics. Increased drying linked with higher temperatures and decreased precipitation has contributed to changes in drought. Changes in sea surface temperatures, wind patterns, and decreased snowpack and snow cover have also been linked to droughts.
• The frequency of heavy precipitation events has increased over most land areas, consistent with warming and observed increases of atmospheric water vapor.
• Widespread changes in extreme temperature have been observed over the last 50 years. Cold days, cold nights, and frost have become less frequent, while hot days, hot nights, and heat waves have become more frequent.
Based on evidence, the IPCC (Working Group II) expresses high confidence that natural systems are affected. Excerpts from the IPCC reports related to effects on natural ecological and biological systems include:
• enlargement and increased numbers of glacial lakes;
• increasing ground instability in permafrost regions and rock avalanches in mountain regions;
• changes in some Arctic and Antarctic ecosystems, including those in sea-ice biomes, and also predators high in the food chain;
• increased runoff and earlier spring discharge in many glacier and snow-fed rivers;
• warming of lakes and rivers in many regions with effects on thermal structure and water quality;
• earlier timing of spring events such as leaf unfolding, bird migration, and egg laying;
• poleward and upward shifts in ranges in plant and animal species;
• shifts in ranges and changes in algal, plankton, and fish abundance in high-latitude oceans;
• increases in algal and zooplankton abundance in high-latitude and high-altitude lakes;
• range changes and earlier migrations of fish in rivers; and
• increasing oceanic acidity.
The reader is directed to the complete IPCC reports for additional detail on observed changes as well as the assessments on projected changes under various future greenhouse gas scenarios.
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