“How might climate change in the 21st century?
When will it happen and how long will it last?”
Climate change projections are carried out using sophisticated computer models to simulate future global and regional climates. The Meteorological Office’s Hadley Centre for Climate Prediction and Research in Exeter has both global and regional climate models which provide the UK’s input to the international and domestic effort to project future climates. Whilst the UK Climate Impacts Programme’s (UKCIP) Climate Change Scenarios for the UK (Hulme et al, 2002) use the Hadley Centre’s output, different models produce different patterns and magnitudes of climate change over the UK. This variation between models is a primary
source of ‘uncertainty’ in projecting future climate. Based on projected seasonal changes in
temperature and rainfall, the Hadley Centre’s global climate model simulates changes close
to the middle of the range for winter but is ‘drier’ in summer than most models.
The progress of climate change in the 21st century depends on future greenhouse gas
emissions which in turn will be determined by how population grows and how economies,
energy technologies and societies develop. These socio-economic factors provide further
sources of uncertainty in projecting climate change. The Intergovernmental Panel on Climate Change (IPCC) has developed a range of projections for future emissions known as
‘emissions scenarios’ based on four socio-economic ‘storylines’ each describing a possible
future world. The storylines are designated A1FI, A2, B1 and B2 and are outlined at Box 4.
Summed over the 21st century, the A1FI storyline – the ‘business as usual’ scenario – has the highest emissions total which is more than twice that of B1, the lowest scenario. The UKCIP has designated these two extremes the ‘high emissions’ and ‘low emissions’ scenarios respectively, which in turn could be considered as worst and best case projections from today’s perspective. This concept of a range of climatic change defined by upper and lower limits is used throughout the next sections on projected climate change.
The principal consequences of climate change are usually described by changes in four key
attributes – temperature, precipitation, extreme weather events and sea level – over a
specified time period. Box 5 outlines the projected global changes in these attributes by the
end of the 21st century.
Emissions of long-lived greenhouse gases like CO2 have a lasting effect on climate. In order
to reduce this impact, emissions need to be reduced to a fraction of what they are today in
order to stabilise greenhouse gas concentrations in the atmosphere. Once stabilisation has
been achieved global average surface temperature is projected to rise for several centuries
as a consequence of the time it takes for the deep ocean to adjust to climate change. Sea
level will also continue to rise for hundreds of years as a result of the thermal expansion of
the oceans. Melt water from ice sheets and glaciers will continue to contribute to sea level
rise for thousands of years after emissions have been stabilised. This sequence of events is
shown graphically at Figure 8.
For projections of climate change in the UK, the Hadley Centre’s regional climate model has
been used based on a 50 km resolution grid. The analysis has been carried out under the
auspices of the UKCIP and climate projections have been made for each of the four IPCC
scenarios over three time periods. These periods are designated the 2020s, 2050s and 2080s which in effect represent the 30-year climate periods of 2011 to 2040, 2041 to 2070 and 2071 to 2100. The key results for the UK for the 2080s are in Box 6.
In a UK context an important feature of the global climate system is the oceanic thermohaline circulation which transports heat from equatorial regions into high latitudes of the North Atlantic. As the Gulf Stream, this flow is responsible for the relatively mild climate of UK. Over the course of this century the Gulf Stream may weaken reducing heat transport from the Tropics into the North Atlantic region. However, this cooling effect is likely to be more than offset by the warming effect of increased greenhouse gases. This has been taken into account in the UK climate projections. The model does not show a complete shut-down by 2100.
The South West Climate Change Impacts Partnership (SWCCIP) has conducted its own
assessment of the regional impact of climate change using Hadley Centre/UKCIP data
(SWCCIP, 2003). A summary of these changes for the period to 2050 is in Box 7.
In addition to the warming effect of the Gulf Stream, the winter climate of the UK is also
affected by a large scale atmospheric circulation known as the North Atlantic Oscillation (NAO). The NAO Index is the difference in pressure between the Azores High and the Icelandic Low and is used as a measure of the ‘westerliness’ of winter weather. When the NAO index is positive (a strong Azores High and deep Icelandic Low) as it has been since the 1980s the North Atlantic storm track is enhanced producing windy but mild and wet winters. A negative index, as experienced during the 1960s, is associated with colder, drier winters with fewer and weaker storms. Simulations show that the future trend is for the NAO Index to become increasingly positive suggesting that UK winters will become more westerly in nature. In addition, the modelling indicates that this enhanced storm track will shift southwards resulting in the strengthening of winter winds over the south of England. Furthermore, winter depressions will become more frequent including deepest ones.
The data used to produce the UKCIP scenarios is available at a 5 km grid resolution and can be used to provide an indication of future temperature and precipitation changes across Devon. Whilst absolute average temperatures and precipitation figures can be derived for each grid square, one has to be wary of attaching any significance to the differences in the climate parameters at this resolution. With this health warning in mind, the UKCIP data has been used to create a contoured change surface as the indicator of climate change at a county level. Temperature and precipitation projections on an annual, summer and winter basis for the high emissions scenario over the three climate periods have been used to produce Figures 9 to 14. The high emissions scenario has been selected as the “business-as-usual” scenario and therefore reflects the likely impacts of proceeding along our present development path. A summary of these projections is at Box 8.
From a World Meteorological Organisation perspective climate is the 30-year average of
principal weather variables such as temperature, precipitation, humidity, soil moisture etc.
Therefore, by definition climate change is a slow process requiring the identification of
statistically significant differences from ever changing 30-year means. In human terms, such changes are almost imperceptible given the daily, seasonal, annual and decadal variability of our weather. However, the evidence from instrumental records going back as far as 1659 (i.e. the Central England Temperature record) indicates that climate change as a result of intervention in the climate system has been taking place for at least half a century. Most of the warming of the past 50 years is attributable to human activity and detection studies consistently find evidence of the anthropogenic signal in the climate record of the last 35 to 50 years (IPCC, 2001c).
As a consequence of the significant time lag in the climate system between cause and effect,
the climate change that is occurring today is a consequence of global emissions released
during the second half of the 20th century. Moreover, we are already committed to the level of climate change projected for the 2020s as a result of historic and present emissions. Any action we take today to reduce emissions will not have an impact on climate until mid 21stcentury and thus the longer we delay the societal shift to a low carbon economy the more likely are our progeny to experience the more extreme and potentially dangerous scenarios of the 2080s.
Finally, we should not ignore the potential for unexpected and rapid climate shifts as the
global climate system seeks to find a new equilibrium. Such events have happened at least
twice in the North Atlantic region since the last Ice Age.