Projected climate change and its impacts

The previous sections have dealt with changes and effects happening today. The rest of the guide concentrates on what is yet to come. 

If current policies to mitigate climate change and related steps towards sustainable development remain unchanged, global GHG emissions will continue to grow over the next few decades. The growth will not be modest, either: The IPCC projects an increase in global GHG emissions from 25 to 90 per cent between 2000 and 2030. (There is a wide margin of uncertainty because of the very different assumptions made in each socio-economic scenario considered by the IPCC). It expects the fossil fuels, oil, coal and gas, to continue to dominate the energy mix till beyond 2030, regardless of the scenario.

Continued GHG emissions at or above current rates will cause further warming and induce many changes in the global climate system during this century that would be larger than those observed during the 20th century. 

Today’s emissions influence the atmosphere for years to come

Even if GHG and aerosol concentrations were kept constant at today’s levels (2000), some anthropogenic warming and sea level rise would continue for many centuries. The climate reacts over long periods to influences upon it; many GHGs remain in the atmosphere for thousands of years.


Backed up by new studies and observations, the IPCC is more certain of the accuracy of the projected warming patterns and other regional climatic effects than it was in the Third Assessment Report. These include wind-pattern changes, precipitation, and some changes in weather extremes and sea ice.

Regional-scale changes include:

  • most warming over land and at the highest northern latitudes, and least over the Southern Ocean and parts of the North Atlantic;
  • contraction of the area covered by snow, increases in the depth at which most permafrost will thaw, and a decrease in the extent of sea ice;
  • increase in the frequency of extremes of heat, heat waves and heavy precipitation;
  • a likely increase in tropical cyclone intensity;
  • a shift towards the poles of storms outside the tropics;
  • increases in precipitation in high latitudes, and likely decreases in most sub-tropical land regions.

The IPCC scenarios (often referred to as the SRES scenarios, for “Special Report on Emissions Scenarios” published by the IPCC in 2000) explore alternative development pathways. They take into account demographic, economic and technological factors and their resulting GHG emissions. The emissions projections based on those different assumptions are widely used in forecasting future climate change, vulnerability and impacts. It is open to anyone to decide which of the different scenarios seems most probable, as the IPCC doesn’t take the risk of attaching any probability to any of them.

Sea level rise

Because our understanding of some important causes of sea level rise is too limited, the IPCC is careful in making predictions: it does not provide a best estimate or an upper limit of the rise to be expected. The range of rise projected for 2090–2099 is from 18 to 59 cm relative to 1980–1999, but this does not include the possible acceleration of ice flow from the polar ice sheets and is therefore not an upper boundary. Other factors contributing to uncertainty in this area include feedbacks between the climate and the carbon cycle and the expansion of ocean water due to warming (“thermal expansion”).

The shrinking of the Greenland ice sheet is projected to contribute to rising sea levels until after 2100. Models suggest a nearly complete melting of the ice sheet.

The Antarctic ice sheet is projected to remain too cold for widespread surface melting: instead, it is expected to grow because of increased snowfall. But a net loss of ice from Antarctica could still be possible, depending on the extent and pace at which ice moves from the land into the sea. 


Since the IPCC’s Third Assessment Report, confidence has increased that a 1–2°C increase in global mean temperature above 1990 levels (about 1.5–2.5°C above pre-industrial levels) poses significant risks to many unique and threatened systems, including many biodiversity hotspots.

These ecosystem changes will be accompanied by shifts in the geographical ranges of both animal and plant species, with mainly harmful consequences for the natural world and for the goods and services which ecosystems provide – like water and food. Many ecosystems are “likely” to exhaust their capacities to cope with changes inflicted on them by climate change and the upheavals associated with it.

Ecosystems will probably find their net carbon uptake peak before the middle of the century and then weaken or even reverse, which would amplify climate change (a positive feedback).

Approximately 20–30 per cent of species are at increased risk of extinction if global average warming exceeds 1.5–2.5°C. As global average temperature increase exceeds about 3.5°C, model projections suggest significant extinctions of 40–70 per cent of known species around the globe. This is one of the irreversible impacts of climate change.3

Food security and human health

The effects of more frequent and intense extreme weather events will cause emergencies and reverse progress towards development. Extreme events coupled with sea level rise, are expected to be mainly adverse for humans. Food is an obvious worry. In higher latitudes there may be an initial slight increase in crop productivity for temperature rises below 3ºC, to be followed by a decrease in some areas. For lower latitudes, productivity may decrease for even small temperature rises. 


The health of millions of people could be at risk from increases in malnutrition, extreme weather, more diarrhoeal diseases, heart and breathing problems caused by climate-induced ground-level ozone, and the spread of some infectious diseases. But there may be some benefits, for instance to those who suffer from the effects of very cold weather. For other issues, effects will be mixed. In some places the geographical range of malaria distribution could contract, while elsewhere it will expand and the transmission season may be changed. Overall, the negative health effects of rising temperatures are expected to outweigh the benefits, especially in developing countries.

Freshwater supply

Climate change is expected to have a crucial impact on all sectors and regions. It would worsen current water stress4 caused by population growth and economic and land-use change.

Regionally, glaciers and snow fields are crucial sources of fresh water. They have undergone recent widespread and severe loss from melting, and this is projected to accelerate this century, reducing water availability and the potential for hydropower (often a feasible alternative to fossil fuel-based electricity generation which reduces CO2 emissions). Climate change is also expected to change the seasonal flows in regions fed by melt water from mountain ranges, like the Hindu Kush, the Himalayas and the inter-tropical Andes. More than a sixth of the world’s population lives in these regions. Two thousand million people depend on the water provided by seven of the major rivers in Asia, all of them originating in the Himalayas.

Changes in precipitation and temperature also affect run-off and water availability. Run-off5 will increase by 10–40 per cent by mid-century at higher latitudes and in some wet tropical areas, and decrease by 10–30 per cent over some dry mid-latitude and tropical regions. Some semi-arid areas, for example around the Mediterranean, in the western USA, southern Africa and north-eastern Brazil, will have less water. Areas affected by drought are projected to increase, threatening food, water, energy production and health by increasing malnutrition and infectious and respiratory diseases. Large regional increases in demand for irrigation water are projected.

The negative impacts on freshwater systems will outweigh the benefits of climate change. Available research suggests a future increase in heavy rainfall events in many regions and some regions where average rainfall is projected to decrease. This will mean an increased flood risk. It is likely that up to 20 per cent of the world’s people will be living in areas where the river flood potential could increase by the 2080s. More frequent and severe floods and droughts will harm sustainable development, rising temperatures will affect fresh water quality, and in coastal areas rising sea levels will mean more saline contamination of groundwater.

Ocean acidification 

The increased amount of carbon dioxide from human activities that has entered the oceans via the atmosphere since about 1750 has made them more acidic, with an average decrease in pH (the measure of the acidity or alkalinity of a solution) of 0.1 units. Increasing atmospheric CO2 concentrations are causing further acidification. Today the average ocean surface pH is about 8.1. Projections suggest a further acidification over this century, leading to a reduction in average global surface ocean pH of between 0.14 and 0.35 units. This progressive acidification is expected to harm marine creatures which form shells, for instance corals, and the species which depend upon them.


Studies published since the 2001 IPCC assessment report allow a more systematic understanding of the timing and extent of impacts linked to different rates of climate change. 

Where are the most affected and who are the most vulnerable?

In all regions, both with low as well as high average incomes, some groups such as elderly people, the poor and young children, and systems or activities are more exposed than others. But there are sectors, systems and regions specially affected, namely the following:

On land:

At risk are areas that are particularly sensitive to the warming effect of climate change, including tundra, boreal forests of the North, and mountain regions. Others will be more affected by the reduced rainfall and changing precipitation patterns, in particular those with a Mediterranean-type environment and tropical rainforests.


On the coast:

Areas facing multiple stresses from climate change include mangroves and salt marshes.

In the ocean:

Coral reefs are ecosystems that are very vulnerable to thermal stress and find it hard to adapt. Increases in sea surface temperature of about 1–3°C are projected to result in more frequent coral bleaching and widespread mortality, unless there is thermal adaptation or acclimatization by corals. They face other climate-change related stresses such as increasing acidity and non climate-related ones (overfishing) in addition. Sea-ice biomes (communities) are also very sensitive to small changes in temperature, when ice turns to water for example.

Other risk sectors include low-lying coasts, water resources in some dry areas and those that depend on melting snow and ice, low-latitude agriculture, and human health in poorer countries. The increased frequency and intensity of extreme weather is “very likely” to worsen other impacts.

Regions expected to be especially hard-hit by climate change include:

The Arctic, because of the high rates of projected warming and its impact on people and the natural world. The thickness and extent of glaciers is expected to decline, and ice sheets and sea ice will also be affected. Invasive species may become a growing problem. 

Africa, with the expected impacts on the continent and its low capacity for adaption. Yields from rain-fed agriculture, for example, could in some countries fall by half by 2020.

Small islands, where people and infrastructure are highly exposed to projected impacts, including sea level rise which is the main problem, as well as reduced rainfall in summer projected for these regions). This would reduce freshwater availability which may leave some unable to meet their demand. Increased rainfall in winter is “unlikely” to compensate this development due to lack of storage and high runoff during storms. In the Pacific, for example, a 10 per cent reduction in average rainfall (by 2050) would lead to a 20 per cent reduction of freshwater on Tarawa Atoll, Kiribati. Furthermore, more alien species may take advantage of higher temperatures to settle on some islands, interfering with the natural ecosystems. 

Asian and African mega-deltas, where large populations have to face high exposure to sea level rise, storm surges and river flooding. In east, south and south-east Asia illness and death from diarrhoeal disease caused by floods and droughts are expected to rise.


Elsewhere Australia and New Zealand are expected to face problems from reduced agricultural productivity and damage to species-rich areas, including the Great Barrier Reef. 

Southern Europe may experience reduced availability of water, mountainous areas across the continent will face glacier retreat and reduced snow cover, leading to higher potential for water shortages, and health risks may increase from heat waves and wildfires. 

Latin America may have less water, as a consequence both of reduced precipitation and retreating glaciers, significant species loss, and by mid-century it may expect the gradual replacement of tropical forest by savanna in eastern Amazonia. Yields of food crops may diminish, exposing more people to the risk of hunger.


North America faces water scarcity, more heat waves, coastal threats and problems for some crops. 

The risk of abrupt or irreversible changes

Climate warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the change. Abrupt climate change on time scales of a decade or so is normally thought of as involving ocean circulation changes (like the meridional overturning circulation – see box). On longer time scales, ice sheet and ecosystem changes may also play a role. 


If a large-scale abrupt climate change effect were to occur, its impact could be quite high. Partial loss of ice sheets on polar land and/or the thermal expansion of seawater over very long time scales could imply metres of sea level rise, with the greatest impacts on coasts, river deltas and islands, implying major changes in coastlines and inundation of low-lying areas. The number of people in the world who live within 100 km of the coast and no more than 100 m above sea level has been calculated at 600 millions to 1.2 billion – between 10 and 23 per cent of the world’s population. Current models project that such changes would occur over very long time scales (millennia) if global temperature were to be sustained at 1.9–4.6°C over pre-industrial levels. Complete melting of the Greenland ice sheet would raise sea level by 7m and could be irreversible.

Meridional overturning circulation

The possibility which continues to fascinate the media, where potentially abrupt change could take place, is in the meridional overturning circulation of ocean water. Ocean currents have specific patterns that are determined by different water densities. Based on current model simulations, it is very likely that the meridional overturning circulation (MOC) of the Atlantic Ocean (the mixing of cold and warm water around the meridian) will slow down during this century (the Gulf Stream bringing warm water to Northern latitudes of Europe is part of this circulation system); nevertheless temperatures in the region are projected to increase. It is very unlikely that the MOC will undergo a large abrupt transition during the 21st century, and longer-term changes in the MOC cannot be confidently assessed. Impacts of large-scale and persistent changes in the MOC are likely to include changes in marine ecosystem productivity, fisheries, ocean CO2 uptake, oceanic oxygen concentrations and terrestrial vegetation.6


2. An ecosystem is a natural unit consisting of all the plants, animals and micro-organisms in a defined area functioning together with all the non-living physical factors of the environment.
3. Many species have evolved to live in a particular and often narrowly-defined environment. As temperatures rise and other effects of a changing climate intensify, their environments are likely to change too quickly for them to either adapt to or to migrate to somewhere more suitable.
4. One definition of water stress is the situation that occurs when a country uses more than 20 per cent of its renewable water supply.
5. The water that falls to Earth and does not infiltrate the ground or evaporate but flows over or through the ground to swell surface or ground water sources.
6. About one-third of anthropogenic CO2 emissions are thought to be entering the oceans, which form the largest active carbon “sink” – absorber – on Earth.

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