climate changes

Scientists are particularly concerned about “tipping points” in the

earth’s systems. These involve processes in which sudden or irreversible

changes occur as systems cross thresholds. Many of them operate at

such a large scale that they are effectively unmanageable by humans

with existing technologies. Four important global tipping points are the

rapid melting of large ice sheets (such as Greenland), large- scale changes

in ocean circulation such as the Gulf Stream, feedback processes where

warming produces more warming, and enhanced warming over the

long run. These tipping points are particularly dangerous because they

are not easily reversed once they are triggered.

Sea-level rise has two major components: thermal expansion and

melting of terrestrial ice. Thermal expansion occurs because water density

changes with different levels of temperature, salinity, and pressure.

On average, as the oceans warm, they will expand, thereby raising sea

level. This part of SLR is well understood and can be accurately modeled.

The oceans have been rising slowly since the last ice age. Current

estimates are that the rate of SLR is approximately 3 millimeters (0.1

inch) per year. Under standard climate-change projections, thermal expansion

will raise the oceans by about 0.2 meters (8 inches) by 2100.

This is only slightly more rapid than the rate of SLR over the twentieth

century.4

The other major component of SLR is melting ice from glaciers and

ice caps, but the estimates here are highly uncertain. What most worries

scientists is the vast quantity of water locked up in the three major

he DICE model to make projections of climatechange

impacts for different scenarios over the coming centuries. The

model includes all sources of SLR, although the dynamics of the ice

caps are very uncertain. The projections are consistent with standard

ocean- climate models but additionally are linked to the economic and

emissions models.