Chapter 1: Observed changes in climate and their effects

The first chapter outlines the observed changes in climate and their effects. The report begins with the bold statement that the climate is undeniably warming, citing the observed increases in atmospheric and ocean temperatures, widespread melting of snow and ice, and rising sea levels, as proof. These statements are exemplified by the fact that eleven of the last 12 years have been the hottest on record since 1850. It is likely that average Northern Hemisphere temperatures over the last 50 years have been hotter than any time over the last 1300 years. The report goes on to explain that the observed warming is global, though higher latitudes have warmed more than the lower latitudes, and land masses have warmed more than oceans.

The report says that there has been an average global temperature increase of 0.74 degrees Celsius over the last 100 years. Due to melting ice and thermal expansion, average sea levels have risen by 1.8mm/yr since 1961 and 3.1mm/yr since 1993, suggesting that sea levels are rising at an increasingly faster rate. In support of this, satellite images show that Arctic sea ice extent has deceased at an average rate of 2.7% per decade since 1978 and mountain glaciers are also retreating.

The warming atmosphere and oceans appear to be impacting rainfall patterns too, observed since 1900, indicted by increased rainfall in east North and South America, northern Europe and northern and central Asia. On the other hand, regions of the Sahel, the Mediterranean, southern Africa and southern Asia have experienced a reduction in rainfall (generally, increase in rainfall in higher latitudes, with a corresponding decrease in rainfall in sub tropical regions). The report concludes that globally there has likely been an increase in the occurrences of drought since the 1970s. It is likely that heat waves and heavy rainfall events have become more frequent, and there is evidence that the number of tropical cyclones has increased, and their storm tracks have increased pole ward. More evidence since the TAR supports this.




The report then moves on to highlight the impacts of increased temperature on various ecosystems. It is important to mention here that 29,000 data series from 75 studies have been assessed by the working group and most interestingly, 89% of the studys' conclusions are consistent with expected impacts of a warming planet. The IPCC has 'high confidence' warming has caused an increase in glacial lakes in snow and ice regions and increased ground instability in permafrost regions. There is 'high confidence' that rivers fed by glaciers and snow melt are experiencing earlier peak spring discharge. In terrestrial ecosystems, there is 'very high confidence' that spring is coming earlier and there has been a pole ward and upward shift in flora and fauna. In marine ecosystems, the increased temperature has affected ice cover, salinity and oxygen levels with a resulting impact on algal, plankton and fish abundance.

Impacts on human and natural environments have also been observed with 'medium confidence'. These include earlier spring planting of crops, increased mortality in the northern hemisphere due to heat waves, shifts in the patterns of infectious disease vectors, and a shrinking of snow cover on ski resorts.