Annex B: Evidence of Climate Change
Climate and climate change
Climate, at a very basic level, is the 'average weather' over a period of time at a particular location. By weather we mean a range of variables such as temperature, rainfall and wind. The standard period over which climate is measured, as defined by the World Meteorological Organisation ( WMO), is 30 years, however, in order to investigate the potential impacts of climate change, we have to look further into the future.
Climate change refers to a significant variation in the average climate, persisting over a long period of time (decades or more). Some climate change may be the result of natural processes ( climate variability), but climate change here refers to changes in global climate that have arisen or are predicted to arise because of human activity that has changed or is expected to change the gaseous composition of the Earth's atmosphere. 61
Greenhouse gases and the greenhouse effect
Changes to the composition of the Earth's atmosphere are caused by the release of GHG emissions from the burning of fossil fuels, such as coal, oil and gas. Greenhouse gases are the elements of the atmosphere, both natural and human-made, that absorb and emit radiation. Water vapour (H 2O), carbon dioxide ( CO 2), nitrous oxide (N 2O), methane ( CH4), and ozone (O 3) are the primary GHGs in the Earth's atmosphere. 62
The natural role of these gases is to trap the sun's heat in the Earth's atmosphere (after this heat has been reflected back off the earth's surface), warming the Earth's surface by over 30ûC to a level at which humans and other living things can survive (see figures 1 and 2 below). This is known as the 'natural greenhouse effect' and each greenhouse gas has a different capacity to cause such warming.
An increase in the concentration of GHGs in the atmosphere to well above the pre-industrial level and the level that would have occurred naturally is known as the 'enhanced greenhouse effect'. It is this enhanced greenhouse effect, caused by the burning of fossil fuels, which is leading to dangerous levels of climate change and which will pose significant risks to existing physical, biological and socio-economic systems.
Evidence of climate change
Absence of the Greenhouse Effect
The Greenhouse Effect
Climate change is undoubtedly a reality. Global temperatures rose by around 0.7ûC over the 20th century and 1998 was the warmest year since records began in 1861 with 2005 almost as warm. All of the evidence points to the primary cause of this warming being an increase in concentrations of GHGs in the atmosphere, as a result of human activities since the industrial revolution. The IPCC concluded in its most recent assessment report 63 that most of the observed warming over the last 50 years was attributable to human activities.
The worst impacts of climate change will be felt by those in the developing world. Rich nations are not immune, however, and Europe has recent first hand experience of the sorts of extreme weather events that will become increasingly common as a result of climate change. For example, widespread flooding devastated several central European cities in 2002, while an unprecedented heatwave led to around 35,000 additional deaths across Europe in 2003. And in 2004 parts of the UK, including Scotland, suffered the crippling effects of flash floods and landslips as a result of several heavy rainfall events. Whilst we cannot say that any one weather event is due to climate change, the frequency of extreme weather events like those mentioned above will increase because of it.
Evidence of human-made climate change
The IPCC, representing the consensus view of thousands of scientists across the world, observed in its Third Assessment Report64 that 'there is now stronger evidence that most of the warming observed over the last 50 years is attributable to human activities'. This evidence includes the following observations:
- levels of CO 2, the main GHG, have risen by more than a third since the industrial revolution and are now rising faster than every before;
- all of the 10 warmest years since records began have occurred since 1990, including each year since 1997;
- winter storms have doubled in frequency in the UK over the past 50 years;
- spring plants are blooming earlier in Scotland (on average three weeks earlier since 1978) 65 and the arrival of migrating birds, the onset of some bird breeding seasons and the emergence of insects have all been observed to occur earlier across much of the middle and high latitudes of the Northern Hemisphere;
- the area of snow cover in the Northern Hemisphere has decreased by 10% since the 1960s and there has been a significant retreat of glaciers in most regions of the world;
- large areas of fragile Antarctic ice shelves have broken-off in recent years;
- Arctic sea-ice, which floats on the Arctic Ocean, has thinned by up to 40% in recent decades in late-summer and has decreased in late-summer extent by 10-15 per cent since the 1950s;
- latest estimates indicate a recent decline in Arctic sea ice area of about 8% in the last decade alone;
- sea level has risen by 1 to 2 mm per year during the 20th century mainly as a result of thermal expansion of the oceans and melting of glaciers and ice shelves;
- coral reef bleaching caused by unusually high sea surface temperatures has increased in frequency. We are also beginning to see an increase in ocean acidity, which has important implications both for marine organisms and the oceans ability to absorb carbon dioxide from the atmosphere;
- in the Arctic, temperatures have risen by up to twice the global average since the mid-1970s and Alaska has warmed especially fast in the last two decades, with dramatic impacts on the environment, plants and animals and human societies; and
- across Europe, several dramatic floods have hit the headlines during the last decade. Possibly the hottest summer in a thousand years set a new UK record high temperature of 38.5 oC near Faversham in Kent on 10 August 2003. It is predicted that by about the middle of this century such a summer may occur every two or three years.
Initial findings of research into changes in climate over the last 40 years in Scotland were published in January 2006. The research, commissioned by the Scotland and Northern Ireland Forum for Environmental Research ( SNIFFER), will be used to produce a handbook of regional changes to be published in spring 2006. Key initial findings include:
- Temperatures have increased in every season and in all parts of Scotland since 1961. This has been the fastest period of warming identified in the analysed record (1914 to 2004).
- Since 1961 daily maximum temperatures have been increasing at a faster rate than minimum temperatures. This is contrary to the trends seen since 1914 in Scotland (and globally) when minimum, or night time, temperatures increased at the faster rate.
- Since 1961 Scotland has become wetter with an average increase of almost 60% in winter months in northern and western Scotland. For the majority of the country there has not been a large-scale significant change in rainfall in summer months although some parts of north west Scotland have become up to 45% drier. Contrary to the national trend, Aberdeenshire has seen little change in precipitation in winter months although this is compensated in this region by significant increase in precipitation in autumn (September-November).
The extent to which changes in temperature over the last 100 years can be attributed to human activities has been studied by looking at patterns of change across the earth's surface and vertically through the depth of the atmosphere. Increasingly complex and integrated climate models predict a characteristic 'fingerprint' pattern of change in response to increasing greenhouse gas concentrations. Statistical analysis shows that this fingerprint can be detected in observed temperature changes, indicating that at least 50% of the increase since 1900 has been caused by human activities; with the remainder being due to natural variations, including changes in the sun's output and volcanic eruptions. The graphs below clearly show how global (since 1860) and northern hemisphere (since 1000) temperature has departed from the average, with a marked rise since the industrial revolution.
Variations in the Earth's surface temperature
(Source: Intergovernmental Panel on Climate Change)
How the world's climate may change in the future
Complex computer models are used to simulate global and regional climates of the past, present and future. Comparing 'model' climates with what has actually happened is fundamental to understanding how climate changes occur and also to improving the models so that they can provide the best possible predictions for future changes. The first graph shows observed global temperature change between 1860 and 2000 as well as a simulation of global temperature change back into the 19th century from the latest climate change model developed by the Met Office's Hadley Centre. The Hadley Centre's model, when driven with all known natural and man-made factors since the 1860s, simulates the observations well. The second graph shows four simulations into the future using the same model and based on four IPCC scenarios for GHG emissions. Each shows a continuing global temperature rise until the end of this century.
A good fit between observed and model-simulated global temperature is only obtained when both natural and human factors are included in the model simulation.
Observed and modelled global temperature trends, 1806-2000
(Source: Hadley Centre)
Global carbon emissions from 2000 to 2100 for the four chosen IPCC emissions scenarios (observed data to 2000)
(Source: Hadley Centre)
Abrupt and dangerous climate change
There have been important recent advances in understanding the climate system. At the Defra-funded International Science Conference 'Avoiding Dangerous Climate Change', held in Exeter in February 2005, it became clear that some possible consequences of increasing global temperature are potentially even more serious than those reported in the IPCC Third Assessment Report, which was published in 2001. Present knowledge indicates that this century may see large-scale, abrupt and high impact changes that we should strive to avoid. These include: acceleration of the melting of the ice sheets covering Greenland and large parts of the Antarctic; the complete removal of Arctic sea-ice each summer, more rapid sea level rise; a significant reduction in the stability of the North-Atlantic Thermohaline Circulation ( THC), leading to changes in the Gulf-stream; the possible release of methane and increases in carbon dioxide from sources such as melting permafrost; and possible drying of the Amazon forest. The final two impacts listed here would increase emissions rates, which in turn would accelerate climate changes.
These and several other candidates for potentially dangerous and rapid changes are beyond the range of current human experience. These are still highly unpredictable, and are the subject of active research. Given that we are already committed to a certain amount of climate change, including a slow and inexorable rise in sea level, a crucial question is how the current upward trend in concentrations of GHGs can ultimately be halted at a level which avoids such dangerous impacts of climate change. In the 1990s the EC proposed that we should aim to limit global temperature rise to 2 oC to avoid dangerous climate change. This equated to atmospheric CO 2 levels below approximately 550 ppm but the IPCC has suggested that a limit closer to 450 ppm might be more appropriate. Since pre-industrial times we have already seen a rise from 270 to 380 ppm (ie 40% of the way to 550 ppm and 60% of the way to 450 ppm or even lower). Also, the longer we delay putting into place measures to reduce emissions, the harder it will be to stabilise at an acceptable level. It is clear then that action is needed now.
Avoiding Dangerous Climate Change
The Avoiding Dangerous Climate Change book, published on 30 January 2006, consolidates the scientific findings of the Exeter conference and gives an account of the most recent developments on critical thresholds and key vulnerabilities of the climate system, impacts on human and natural systems, emission pathways and technological options of meeting different stabilisation levels of greenhouse gases in the atmosphere.
The book focuses on three crucial questions:
1. For different levels of climate change what are the key impacts, for different regions and sectors, and for the world as a whole?
2. What would such levels of climate change imply in terms of greenhouse gas stabilisation concentrations and emission pathways required to achieve such levels?
3. What technological options are there for achieving stabilisation of greenhouse gases at different stabilisation concentrations in the atmosphere, taking into account costs and uncertainties?
The text of the book is available on Defra's website and the book can be ordered online via the Cambridge University Press website.
Will the Gulf Stream collapse?
A key question in climate research concerns the stability of the thermohaline circulation, a system of large-scale currents including the Gulf Stream in the North Atlantic Ocean, which carries heat from the tropics to higher latitudes as cold salty water sinks near the pole, drawing warm water north-eastwards. The arm of the Gulf Stream which flows up the west coast of Scotland (the North Atlantic Drift) is responsible for keeping our climate temperate.
Recent observations have shown a reduction in the amount of salt in the sea water deep in the north-west Atlantic, and this has been interpreted by some as an early sign of a weakening thermohaline circulation.
The climate model run by the Hadley Centre - part of the Met Office - shows that the observations are, in fact, consistent with a slight strengthening of the thermohaline circulation since the 1960s. Nevertheless, the model predicts that in future it will weaken somewhat as a result of global warming.
Hadley Centre models suggest a reduction in the strength of the Gulf Stream by as much as a quarter, but not a collapse. However, even with this reduction in the Gulf Stream, the net result of climate change is expected to be a warmer Europe.
(Source: The Met Office, see: www.metoffice.com )