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The impact of a global temperature rise of 4ºC (7 ºF)

The impacts of climate change will be widespread across the globe. In order to understand more about what the human impact of high-end climate change might be, and therefore what would happen if a successful agreement can not be reached at Copenhagen, the UK's Met Office Hadley Centre has produced a map outlining some of the impacts that may occur if the global average temperature rises by 4 °C (7 °F) above the pre-industrial climate average. The map represents the latest peer-reviewed science on the impacts.  

Using the map: This interactive version of the 4 degree map allows you to select which impacts you want to see, zoom on specific geographies and access more information about the science behind the map.

Click on the map to activate the controls


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Impacts on human activity

The projection shown on the map was generated using Met Office Hadley Centre's HadCM3 model, and shows that an average rise of 4 ºC (7 ºF) will not be spread uniformly across the globe.  

To create the map the model was run many times, for two different greenhouse gas emission scenarios. These are two of the socio-economic scenarios developed by the Intergovernmental Panel on Climate Change (IPCC), representing greenhouse gas emissions (and other emissions such as aerosols) for future development without active mitigation against climate change. The two used here are known as A1B and A1FI. Out of a total of 34 model runs, 23 runs showed the global average temperature rising above 4 ºC (7 ºF) by the end of the century. Emissions are currently running close to the A1FI scenario and for this scenario all but one the model runs reached a 4 ºC (7 ºF) global average temperature rise in this time.  The range was 3.2 ºC (5.8 ºF) to 6.7 ºC (12.1 ºF). The output of each of these 23 simulations was averaged for the decade when they crossed the 4 ºC (7 ºF) threshold. This is the projection shown. This projection does not therefore include information about timescales of climate change or the likelihood of reaching 4 ºC (7 ºF).

The impacts shown on the map are only a selection of those that may occur and are focused primarily on the effect of climate change on human activity. Where numbers of people are quoted, the underlying assumption is that population changes will follow the population projections used by the IPCC to develop the A1B emission scenario. This means that the impacts are consistent with each other.

However, the evidence for the impacts shown comes from a number of different research studies. In some cases the impacts have been driven by different model projections, although all the climate data used is consistent with a 4 ºC (7 ºF) global average-temperature rise.

Adaptation options to the impacts shown on the map may or may not always be feasible. For this reason no assumptions are made about the adaptive capacity of the countries and regions affected. Instead, the values described as 'up to' indicate the worst case in a range of outcomes that include model uncertainty and behavioural responses.

The Amazon forest

With high levels of climate change, large areas of the Amazon forest could be lost through either drought stress on vegetation or the uncontrolled spread of fire. This depends largely on whether rainfall will decrease in Amazonia. While some climate models suggest rainfall may increase, some of the more realistic models project severe drying in the Amazon, increasing the risk of major droughts.

Carbon cycle

The 20th Century rise in CO2 concentration was only 40-50% of the actual rate of emissions, because the rest was absorbed by the world's ecosystems and oceans.  This process may be damaged by climate change, so that the impact of emissions on atmospheric concentrations could be greater in the future. At 4 ºC (7 ºF) increase in global average temperature, the proportion of CO2 emissions remaining in the atmosphere could rise to as much as 70%. The longer emission cuts are delayed, the less effective they will be in stabilising CO2 in the atmosphere.

Agriculture

Climate change directly affects crop productivity and food production. Changes in the regional differences in climate patterns may widen production and consumption gaps between the developed and developing world. Current assessments are mainly limited to alterations in mean climate, but extreme weather or glacial retreat would potentially accelerate declines in productivity further.

Agricultural yields are expected to decrease for all major cereal crops in all major regions of production, once the global average temperature increases beyond 3 ºC. For some crops the yield could decrease by over 20% at low latitudes, where the impact will be greatest. This could result in tens to hundreds of millions of additional people (roughly a 10-20% increase), at risk from hunger. Most of this increase is expected in Sub-Saharan Africa, and in some parts of south Asia and Central America, particularly for child malnutrition. For the population at 2050 the increase in the number of malnourished children could be as high as 24 million.

Water availability

A rise in global average temperature of 4 ºC (7 ºF) would have a substantial effect on river flows and the availability of water.

For the population rise at 2080, without climate change, just over 3 billion people, out of a global population of 7.5 billion, could be living in areas with limited per capita water availability (less than 1000m3/person/year).

By reducing river run-off, climate change could mean that significantly less water was available to approximately 1 billion of these people (range 0.4 to 2 billion), substantially increasing the pressure of managing water supplies.  In addition, as glaciers retreat, communities relying on glacier melt-water will also come under further threat.

Sea-level rise

Sea-level rise is an inevitable consequence of increasing global temperatures. Low-lying coastal areas will become more vulnerable to flooding and land loss. As these areas often have dense populations, important infrastructure and high value agricultural and bio-diverse land, significant impacts are expected. At the beginning of the 21st Century, an estimated 600 million people live no more than 10 metres above present sea level.

South and East Asia have the highest populations living in low-lying deltas, but small islands are also vulnerable from sea-level rise and storm surges. Flooding from sea-water would cause loss of land, crops and freshwater supplies, posing a risk to stability and security. For some, forced migration will be inevitable.

Temperature rises

An average global temperature rise of 4 ºC (7 ºF) is not uniform as oceans heat more slowly than the land, and high latitudes, particularly the Arctic, will have larger temperature increases. The temperature of the very hottest days will also increase and many areas of high population density will see a larger change in extreme high temperatures. This will have a significant impact on health. Temperature rises will impact water availability, agricultural productivity, the risk of fire, the melting of ice sheets and the thawing of permafrost. Commercial activity will also be affected by loss of productivity in hotter conditions or the cost of maintaining cooler working environments.

Heat-related mortality and other adverse health impacts are likely to increase considerably, even when acclimatisation, adaptation and fewer cold-related deaths are taken into account. In 2003 for instance, the European heat wave was responsible for around 35,000 additional deaths.