The Basics of Climate Change

 

14

How fast is sea level rising?

Long-term measurements of tide gauges and recent satellite data show that global sea level is rising, with the best estimate of the rate of global-average rise over the last decade being 3.6 mm per year (0.14 inches per year). The rate of sea level rise has increased since measurements using altimetry from space were started in 1992; the dominant factor in global-average sea level rise since 1970 is human-caused warming. The overall observed rise since 1902 is about 16 cm (6 inches).

This sea level rise has been driven by expansion of water volume as the ocean warms, melting of mountain glaciers in all regions of the world, and mass losses from the Greenland and Antarctic ice sheets. All of these result from a warming climate. Fluctuations in sea level also occur due to changes in the amounts of water stored on land. The amount of sea level change experienced at any given location also depends on a variety of other factors, including whether regional geological processes and rebound of the land weighted down by previous ice sheets are causing the land itself to rise or sink, and whether changes in winds and currents are piling ocean water against some coasts or moving water away.

The effects of rising sea level are felt most acutely in the increased frequency and intensity of occasional storm surges. If CO2 and other greenhouse gases continue to increase on their current trajectories, it is projected that sea level may rise, at minimum, by a further 0.4 to 0.8 m (1.3 to 2.6 feet) by 2100, although future ice sheet melt could make these values considerably higher. Moreover, rising sea levels will not stop in 2100; sea levels will be much higher in the following centuries as the sea continues to take up heat and glaciers continue to retreat. It remains difficult to predict the details of how the Greenland and Antarctic Ice Sheets will respond to continued warming, but it is thought that Greenland and perhaps West Antarctica will continue to lose mass, whereas the colder parts of Antarctica could gain mass as they receive more snowfall from warmer air that contains more moisture. Sea level in the last interglacial (warm) period around 125,000 years ago peaked at probably 5 to 10 m above the present level. During this period, the polar regions were warmer than they are today. This suggests that, over millennia, long periods of increased warmth will lead to very significant loss of parts of the Greenland and Antarctic Ice Sheets and to consequent sea level rise.

 

15

What is ocean acidification and why does it matter?

Direct observations of ocean chemistry have shown that the chemical balance of seawater has shifted to a more acidic state (lower pH). Some marine organisms (such as corals and some shellfish) have shells composed of calcium carbonate, which dissolves more readily in acid. As the acidity of sea water increases, it becomes more difficult for these organisms to form or maintain their shells.

CO2 dissolves in water to form a weak acid, and the oceans have absorbed about a third of the CO2 resulting from human activities, leading to a steady decrease in ocean pH levels. With increasing atmospheric CO2, this chemical balance will change even more during the next century. Laboratory and other experiments show that under high CO2 and in more acidic waters, some marine species have misshapen shells and lower growth rates, although the effect varies among species. Acidification also alters the cycling of nutrients and many other elements and compounds in the ocean, and it is likely to shift the competitive advantage among species, with as-yet-to-be-determined impacts on marine ecosystems and the food web.

 

16

How confident are scientists that Earth will warm further over the coming century?

Very confident. If emissions continue on their present trajectory, without either technological or regulatory abatement, then warming of 2.6 to 4.8 °C (4.7 to 8.6 °F) in addition to that which has already occurred would be expected during the 21st century.

Warming due to the addition of large amounts of greenhouse gases to the atmosphere can be understood in terms of very basic properties of greenhouse gases. It will in turn lead to many changes in natural climate processes, with a net effect of amplifying the warming. The size of the warming that will be experienced depends largely on the amount of greenhouse gases accumulating in the atmosphere and hence on the trajectory of emissions. If the total cumulative emissions since 1875 are kept below about 900 gigatonnes (900 billion tonnes) of carbon, then there is a two-thirds chance of keeping the rise in global average temperature since the pre-industrial period below 2 °C (3.6 °F). However, two-thirds of this amount has already been emitted. A target of keeping global average temperature rise below 1.5 °C (2.7 °F) would allow for even less total cumulative emissions since 1875.

Based just on the established physics of the amount of heat CO2 absorbs and emits, a doubling of atmospheric CO2 concentration from preindustrial levels (up to about 560 ppm) would by itself, without amplification by any other effects, cause a global average temperature increase of about 1 °C (1.8 °F). However, the total amount of warming from a given amount of emissions depends on chains of effects (feedbacks) that can individually either amplify or diminish the initial warming.

The most important amplifying feedback is caused by water vapour, which is a potent greenhouse gas. As CO2 increases and warms the atmosphere, the warmer air can hold more moisture and trap more heat in the lower atmosphere. Also, as Arctic sea ice and glaciers melt, more sunlight is absorbed into the darker underlying land and ocean surfaces, causing further warming and further melting of ice and snow. The biggest uncertainty in our understanding of feedbacks relates to clouds (which can have both positive and negative feedbacks), and how the properties of clouds will change in response to climate change.

Other important feedbacks involve the carbon cycle. Currently the land and oceans together absorb about half of the CO2 emitted from human activities, but the capacities of land and ocean to store additional carbon are expected to decrease with additional warming, leading to faster increases in atmospheric CO2 and faster warming. Models vary in their projections of how much additional warming to expect, but all such models agree that the overall net effect of feedbacks is to amplify the warming.