Projections: Temperature, ocean acidification and sea level rise
Temperature: Earth’s average surface temperature has increased over the 20th century by about 0.6 oC. Although this number may at first seem quite small, it is likely to have been the largest increase for any century in the last 1,000 years. Moreover, evidence from tree ring records, used to reconstruct temperatures over this period, suggests that the 1990s was the warmest period in a millennium. According to the Intergovernmental Panel on Climate Change (IPCC), observed temperature rises since 1990 are about 0.2oC per decade. For the next two decades, a warming of about 0.2oC also is projected. Based on six potential future emissions scenarios (see figure below), the IPCC estimates that temperatures by the end of the 21st century will increase by 1.8 oC - 4.0 oC (this is a “best estimate” range). Perhaps more striking, this temperature increase would result in a sea level rise of between 28-43 centimeters.
Although anthropogenic (human-caused) warming has just become evident over the past fifty years or so, the amount of greenhouse gases already released into the atmosphere will impact the climate system well into the future. According to the IPCC, even if the concentrations of all greenhouse gases and aerosols had been kept constant at year 2000 levels, a further warming of about 0.1oC per decade would be expected. Continued greenhouse gas emissions at or above current rates will cause further warming and induce many changes in the global climate system—many of which will be larger than those already observed during the 20th century.
Ocean acidification: Because of air-sea interactions and the chemistry of seawater, increasing atmospheric CO2 concentrations leads to a decrease in the pH of ocean surface waters. In other words, increased atmospheric CO2 will cause the oceans to become more acidic.
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In a more acidic ocean, calcium carbonate (the foundation of the shells and skeletons of many aquatic organisms like corals and plankton) starts to dissolve, making it difficult for these organisms to survive. Global surface ocean pH is expected to decline between 0.14 and 0.35 units over the 21st century, adding to the present decrease of 0.1 units since pre-industrial times. For more detail on the ocean carbonate system, see the University of Delaware’s CMES CO2 site here. To view the Royal Society’s report on Ocean Acidification, click here.
A small drop in pH has been seen in the oceans thus far, and predictions indicate a larger pH drop in the immediate future with projected continual increase in carbon emissions.The figure to the left shows modeled changes in ocean pH from before the Industrial Era into the future (lower panel). Note depth scale on the y-axis (vertical). The upper two panels show the history of anthropogenic CO2 emissions and atmospheric CO2 concentration, respectively, over the same time scale. Modeling shows that the surface ocean pH decrease will reach its maximum magnitude of -0.77 units by 2250; there is little drop in deep ocean pH until about 2500. (Source: Caldiera and Wickett, 2003)
Sea level rise. Sea level has already risen due to global warming, and continued increase of global greenhouse gas emissions will only exacerbate the trend. The two main reasons for global sea-level rise are: (1) thermal expansion of seawater and (2) increased ocean mass.
Thermal expansion: As ocean waters warm, the water molecules expand, causing an increase in the ocean’s volume. This, in turn, causes sea level to rise.
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From 1955 to 1995, ocean thermal expansion is estimated to have contributed about 0.4 mm per year to sea level rise. For the 1993 to 2003 decade, when the best data are available, thermal expansions is estimated to have contributed about 1.6 mm per year, or about 50 percent of the observed sea-level rise of 3.1 mm per year over that period.
Increased Ocean Mass: An increase in ocean mass occurs from the melting of land-based sources of ice, such as glaciers, ice caps, and the ice sheets of Greenland and Antarctica. The melting of glaciers and ice caps (excluding the glaciers surrounding Greenland and Antarctica) is estimated to have contributed about 0.3 mm per year from 1961 to 1990, increasing to about 0.8 mm per year from 2001–2004.
But what are the predicted levels of sea level rise for the future? In the figure below, the IPCC’s initial projections—from the 2001 Assessment Report—are shown by the red lines and grey shading. Updated projections from the most recent report are shown by the blue bars plotted at year 2095. The dark blue bar is the range of model projections and the light blue bar has the upper range extended to allow for the potential contribution from the Greenland and Antarctic ice sheets.
The inset shows the observed sea levels from tide gauges (orange) and satellites (red). Note that these are tracking along the upper bound of the IPCC 2001 projections since the start of the projections in 1990.
Other projected changes: According to the IPCC, snow cover is projected to decrease, with an increase in the thaw depth over most permafrost regions. Sea ice is projected to shrink in both the Arctic and the Antarctic. In some projections, arctic late-summer sea ice disappears almost entirely by the latter part of the 21st century. A new study using computer models of climate and ice loss predicts that summer Arctic sea ice could be gone by 2040 (Holland, Bitz and Tremblay 2006). Click here to view a video of this prediction.
In addition, increases in the amount of precipitation may occur in high latitudes, while decreases are likely in most subtropical land regions. There is a high probability that hot extremes, heat waves, and heavy precipitation events will continue to become more frequent, and future tropical cyclones (typhoons and hurricanes) will likely become more intense.