Limits to warming

Ecosystems

As far as ecosystems are concerned, the effects increase with increasing warming, and there seems to be no threshold effect. If the gradual temperature increase remains limited to 1 degree Celsius since the end of the pre-industrial era, the effects on ecosystems will remain limited, but serious effects on highly vulnerable ecosystems (such as coral reefs) cannot be ruled out. An increase in the global mean temperature of around 1 degree Celsius would probably lead to large-scale bleaching of coral reefs. If it is decided that no valuable ecosystems of substantial size may be lost, we will arrive at a gradual increase in the range of 1-2 degrees. If warming increases beyond 2 degrees, the risks will increase further. This means that a large number of species would become threatened with extinction, that biodiversity in protected areas would decline or that entire ecosystems would disintegrate.

Food production

In terms of food production, a global temperature increase of 3-4 degrees Celsius would not be expected to lead to a global loss of food production, but there is a significant regional risk of productivity losses from as little as a 1 degree increase. This could lead to an increase in the number of people suffering from food shortages. The effects of climate change on food production will highly depend on the ability to adapt, by means of crop selection, (bio )technology, possible relocation of the agricultural infrastructure etc. It is uncertain whether sufficient possibilities exist to do so. If the political will is to have no significant regional risks of (additional) food shortages, we will arrive at a figure that must stay below 2 degrees.

Sea level rise

The slow response of the oceans to the increasing surface temperature as a result of increasing concentrations of greenhouse gases means that part of the sea level rise can no longer be avoided either in this century or thereafter. Scientific uncertainties about the relationship between increasing temperature and the magnitude of future sea level rise mean that this is still difficult to quantify. The possibly larger contributions to sea level rise from the melting of the Greenland and West-Antarctic ice sheets, both this century and in the very long term (post-2100), increase this uncertainty. Still, very little is known about these melting processes. If it is decided to limit the risks of sea-level rise over the very long term (after 2100) by preventing the irreversible melting of the Greenland ice sheet, it would be desirable to limit the mean temperature rise to 1-2 degrees Celsius.

Arctic sea ice

If it is decided to limit the effect of global average warming over the Arctic to prevent the loss of sea ice in summer, global mean warming of, at most, 2 degrees Celsius must be maintained. This degree of warming would translate to an increase of 4-8 degrees in the Arctic.

Thermohaline Circulation

It is currently difficult to indicate the global mean temperature ranges above which the risk of a shutdown of the thermohaline circulation will be high. Provisionally, it appears that this will only happen with a temperature increase of more than 4-5 degrees Celsius. However, present measurements already reveal a weakening of the North Atlantic Circulation, while a number of scenarios arrive at this temperature rise at the end of the present century. So, this is a point of particular attention.

Compared with Third Assessment Report IPCC

Compared with the last IPCC report (2001), more is now known about a number of effects of climate change and several risks are now assessed as being more serious, as outlined below.

• The latest literature studies reveal that a number of vulnerable ecosystems, such as coral reefs and marine ecosystems, are already experiencing the influence of climate change.
• The IPCC report had already pointed out the risks associated with the melting of the Greenland ice sheet if the local temperature were to rise by more than 2.7 degrees Celsius, stating that this would probably be irreversible. Since then there have been indications that the West-Antarctic and Greenland ice sheets may melt faster than had originally been supposed and may contribute considerably to global sea level rise even in the present century.
• The effect of global mean temperature rise appears to be amplified in the Arctic, which has consequences -not just for the local population, but also globally- due to the higher contribution to sea level rise (by melting of the Greenland ice sheet) and the possible release of methane caused by melting permafrost.
• The 2001 IPCC report stated that global warming may play a role in weakening or possibly even a shutdown of the thermohaline circulation. More certainty about this risk has now been obtained, as revealed by the fact that recent literature contains statements about the risk.