Impact of 4°C global temperature rise
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 three degrees Celsius. 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 1020% increase), at risk from hunger.
Most of this increase is expected in sub-Saharan Africa, and some parts of south Asia and central America, particularly for child malnutrition. For the population in 2050, the increase in the number of malnourished children could be as high as 24 million. A rise in the global average temperature of four degrees Celsius would have a substantial effect on river flows and the availability of water.
The Intergovernmental Panel on Climate Change (IPCC) report presented five scenarios based on varying levels of carbon dioxide (CO2) and other greenhouse gas emissions. Under the high and very high emissions scenarios outlined in the report, global heating is predicted to reach 3.6°C and 4.4°C above pre-industrial levels respectively, by the end of the century.
Even in the intermediate scenario global warming of 2°C would be extremely likely to be exceeded. The report found that every additional 0.5°C of global warming causes discernible increases in the intensity and frequency of heatwaves, heavy rainfall, droughts and extreme weather events.
For the population rise by 2080, without climate change, just over three billion people, out of a global population of 7.6 billion, could be living in areas with limited per capita water availability (less than 1 000 cubic meters/person/year). By reducing river run-off, climate change could mean that significantly less water was available to approximately one billion of these people, 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 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 biodiverse land, significant impacts are expected. At the beginning of the 21st century, an estimated 600 million people live no more than 10 metres above the present sea level. South and East Asia have the highest populations living in low-lying deltas, but small islands are also vulnerable to sea-level rise and storm surges.
Flooding from seawater would cause loss of land, crops and freshwater supplies, posing a risk to stability and security. For some, forced migration will be inevitable. 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 the impact of emissions on atmospheric concentrations could be greater in the future. At a four-degree Celsius 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. Furthermore, an average global temperature rise of 4°C 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 extremely high temperatures. This will have a significant impact on health.
Temperature rises will impact water availability, agricultural productivity, the risk of fire, the meltingoficesheetsandthethawing of permafrost. Commercial activity will also be affected by the loss of productivity in hotter conditions or the cost of maintaining cooler working environments. Heatrelated 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.
Drought events occur twice as frequently across southern Africa, South-East Asia and the Mediterranean basis. The IPCC’s new report makes clear that the warming we’ve experienced to date has made changes to many of our planetary support systems that are irreversible on a timescale of centuries to millennia.
The oceans will continue to warm and become more acidic. Mountain and polar glaciers will continue melting for decades or centuries. Richard Betts said “If we allow the global average temperature to rise by 4°C then the science tells us that we can expect severe and widespread impacts worldwide. We can take action now to prevent that temperature rise and avoid those global impacts.’’