Projected Climate Change and Sea Level Rise

In the latest IPCC report the projected levels of global warming in the absence of efforts to reduce emissions are not dramat ically different from those made in earlier reports: warming by 2100 is projected to be in the range of 1.1-6.4 degrees Celsius above the average in the 1980-99 period. Given that emissions, warming, and sea level rise during the current decade have all been at the upper end of projected ranges, it would be prudent to assume that the likely warming in the absence of major emission reductions over the next century will be toward the mid or upper end of the range projected by the IPCC.4

The main reference point for greenhouse gas concentrations and temperature increases is typically preindustrial times. This is usually taken as 1750, so preindustrial CO2 concentration levels are given as 278 parts per million (ppm) CO2. Increases in greenhouses gases (taken together as CO2-equivalent (CO2eq) concentrations) are generally related to this number. A doubling of GHG concentrations means an increase that is equivalent to the effect of about 556 ppm CO2 (often just rounded to 550 ppm CO2).

As far as possible, global temperature increases here are referred to as increases above the preindustrial level. Given that a global instrumental temperature series only exists for the period after 1850, the preindustrial period is defined as the 30-year average from this year. (The average global mean temperatures between 1750 and the 1850s were quite similar, so this is considered satisfactory.) From the 1850s to the five-year period ending in 2007, global mean temperature increased by more than 0.7 degrees Celsius. In the IPCC report, projections are often stated with respect to the period 1980-99 (with 1990 used as the midpoint), which was a bit over 0.5 degrees Celsius warmer than the preindustrial period. So the IPCC's projected increase for the twenty-first century of 1.1-6.4 degrees Celsius above 1980-99 levels would be about 1.6-6.9

A Safe Landing for the Climate

Box 2-1. Preventing Dangerous Climate Change

The guiding principles of international efforts to deal with climate change were established in 1992 in the United Nations Framework Convention on Climate Change, which was adopted in Rio de Janeiro at the Earth Summit:"The ultimate objective of this Convention and any related legal to achieve...stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. Such a level should be achieved within a time-frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner."

This is a powerful statement, as it contains a legally binding requirement to prevent dangerous changes. In practice, however, exactly what this means remains undefined in international law.The article is ambiguous, as it leaves open core questions such as dangerous to whom and to what. What if food production increases in some regions due to global warming and increased CO2 concentration, as is projected for the northern high latitudes, but decreases perhaps dangerously in other regions, as is projected for low-latitude tropical regions such as Africa? Is that dangerous within the meaning of the convention? Answering such questions is fundamental to the development of a fair and equitable global approach to climate change.

While most attention in debates about climate change has focused on changes in climate, it needs also to be noted that under Article 2 "dangerous anthropogenic interference" relates to the climate system as whole: changes in ocean acidity due to human-induced CO2 increases that result in adverse changes in the oceans and marine ecosystems could also be deemed dangerous. University ofToronto clima-tologist Danny Harvey has pointed that there are important differences between terms such as dangerous interference and dangerous climate change. (For simplicity's sake, however, these are used synonymously in this chapter.)

Decisions as to what is "dangerous" fundamentally affect the rate, timing, and scale of emissions reductions required regionally and globally in the coming years and decades. If "dangerous interference" is considered to begin only once the global average temperature exceeds 4 degrees Celsius above the preindustrial level, then it will be hard to justify urgent and stringent mitigation action in the next 10-30 years, as greenhouse gas emissions would not need to peak until well after the 2050s before dropping. If, on the other hand, warming of more than 2 degrees above preindustrial is deemed dangerous, then there is acute and urgent emphasis on near-term emission actions leading to large global emissions reductions of 80 percent or more by 2050.

Source: See endnote 3.

degrees Celsius above preindustrial level. Since 1980-99, the climate system has already warmed about 0.25 degrees Celsius.5

For projected sea level rise the IPCC was unable to estimate fully all the contributions of global warming, as numerical computer models of the ice sheets of Greenland and Antarctica cannot yet adequately project the effects. So the range of sea level rise estimated by the IPCC—between 0.18 and 0.59 meters by 2100 above 1980-99 levels—was heavily qualified, given that the possible future rapid loss of ice from Greenland and Antarctica could not be quantified. The already observed rapid loss of ice in response to recent warming of the atmosphere and ocean around Greenland and West Antarctica indicates that these ice sheets could be more vulnerable to warming than implied by ice sheet models and hence could add significantly to future sea level rise. As a consequence, the IPCC could not give a "best estimate" or upper bound for sea level rise.6

After the writing of the IPCC science

A Safe Landing for the Climate report was completed, Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research projected future sea level rise based on the observed relationship between sea level and temperature over the last century. Using a similar range of emission and climate projections, he estimated a sea level rise in the range of 0.5-1.4 meters above 1990 levels by 2100. More recent work indicates that the increase during this century could be even higher. In short, the evidence points to a likelihood of meter-scale sea level rise by 2100, well above the top end of the range quantified by the IPCC. Thus, much larger risks to coastal zones and small islands seem likely during this century than had previously been estimated.7

There is much greater confidence now than in earlier IPCC assessments in the regional changes that can be expected in a warmer world. Warming will be greatest in the high north and in the interiors of the continents. Reduction in snow cover, a thawing of permafrost, and decreases in the extent of sea ice in both hemispheres can be expected.8 Weather extremes and water availability are two of the most important projections in terms of impacts on human and natural systems. More-frequent heat extremes and heat waves, more-intense tropical cyclones, and heavier precipitation and flooding can be expected in many regions. Recent projections confirm that extreme high surface temperatures will rise faster than global warming and indicate a 10 percent chance of "dangerously high" surface temperatures over 48 degrees Celsius every decade in much of the world by 2100 if the global temperature exceeds 4 degrees Celsius above the prein-dustrial level.9

Precipitation can be expected to decrease in most subtropical land regions but to increase in the high latitudes. The IPCC assessment found with "high confidence that many semi-arid areas (e.g. Mediterranean basin, western United States, southern Africa and northeast Brazil) will suffer a decrease in water resources due to climate change." By the 2050s it is projected that there will be less annual river runoff and water availability in dry regions in the mid-latitudes and tropics but an increase in high-latitude regions and in some tropical wet areas.

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