what is climate change?

The term ‘climate’ refers to a representation of average weather conditions, which is often described in terms of temperature and precipitation but can also include wind and other derived variables such as growing degree days. Weather is what we experience day to day. The climate of the Earth is not static; it has changed over geological and recent historic time periods, and will continue to do so into the future. It is this future anticipated change which is usually meant when the term ‘climate change’ is used.

The climate change that will occur over the next 30 to 40 years has already been largely determined by past and present emissions of greenhouse gases. Once emitted into the atmosphere these gases can last for a long time and so can influence the climate system into the future. This means that even if emissions are vastly reduced with immediate effect, the climate will continue to change in response to the legacy of emissions and adaptation measures will still be needed.

What is Causing Climate Change?

Climate change is caused by a combination of both natural and human-induced factors.  The Earth’s climate varies naturally as a result of interactions between the atmosphere and the ocean, changes in the Earth’s orbit, fluctuations in energy received from the sun, and volcanic eruptions.  Human activities also contribute towards climate change through emissions and change sin land use. A majority of scientists and Governments now believe that these activities are a major cause of climate change.

The main human influence on global climate change is now widely recognised to be emissions of greenhouse gases, such as carbon dioxide (CO2) and methane (CH4).  There is now a greater concentration of CO2 and methane in the atmosphere than at any time in the last 650,000 years, with anything from 6.5 billion tonnes (UKCIP, 2006) to 25 billion tonnes (BBC, 2006) quoted as being added annually, mostly through the burning of coal, oil and gas.  This rate of emissions generated also seems to be increasing at a faster rate than in the past.

The so-called greenhouse gases are transparent to shorter wavelengths of light coming in to atmosphere, but impede the outgoing longer wavelengths of light or heat. Increases in the concentrations of CO2 and other greenhouse gases act like a blanket and trap more heat in the atmosphere which leads to increases in global temperature.

There has been some speculation that the threshold of CO2 concentrations in the atmosphere beyond which irreversible climate change will occur, is around 400 parts per million (ppm). According to recent measurements, the atmospheric concentration of CO2 has increased to 380 ppm. The figure to the right shows the changes in global concentrations of CO2 in the post-industrialisation era.

There exists a risk that natural and human responses to climate change further exacerbate the problem in the form of ‘feedback’ mechanisms. For example, as the temperature increases, so the demand for energy-hungry air conditioning system increases, releasing further greenhouses gases into the atmosphere. Similarly, water vapour is one of the most potent greenhouse gases and as temperatures increase so more is retained in the atmosphere through processes such as increased evaporation and evapotranspiration.

Changes in the Global Climate

Many aspects of the Earth’s climate are changing, but it is changes in temperature (so-called global warming) that receive the most attention, principally because it is this aspect which has shown the most marked changes over recent decades. Additionally the modelling of future climate temperatures is generally more advanced than for other climate variables.

The Met Office’s Hadley Centre for Climate Prediction and Research has prepared the figure to the right, showing variations in global near-surface land temperature since 1860 relative to the average temperature between 1961 to 1990. This clearly shows a marked increase in temperature over this time period. Translating recent historic trends into the future projections, most scientists believe that the Earth will warm between 1.1oC and 6.4oC by 2100 (IPCC 2007).

 

This figure, also from the Met Office’s Hadley Centre is based on the assumption that current emissions trends will continue, with moderate economic growth and few measures to reduce emissions put in place. It compares the climate from 1960 to 1990 with the projected climate of between 2070 to 2100. This scenario suggests that the greatest rises in average surface air temperature by the 2080s in northern polar regions, India, Africa and parts of South America. Other scenarios produce different but generally consistent projections.

One of the most concerning consequences of global temperature increases is the melting of the Earth’s sea ice.

For example, there has been a marked and pervasive change in the Arctic sea ice over the last few decades, as shown in this figure depicting the thickness of various Arctic sea ice. The remarkable pace of change that is being experienced is demonstrated by considering that an area of Arctic sea ice five times the size of the UK landmass has been lost since 1979.

In addition, the extent of global snow cover has shrunk by 10% since the 1960s and mountain glaciers have also retreated. A negative feedback mechanism exists following ice melt whereby newly-exposed land absorbs more heat (which would have been reflected by the ice) and hence further contributes to near-surface land temperature warming (this is called the ice albedo effect). Similarly as ice sheets melt so the exposed oceans become a darker surface, leading to greater heat absorption, increased temperature and further ice melt.

One of the consequences of climate change is rising sea levels. This is caused as the oceans expand due to increased water temperature and glacier ice, ice sheets and mountain snow melt in response to increasing temperatures. The IPCC quotes global sea level rises between 10 and 20cm during the 20th Century and further projects a rise of between 0.18 m and 0.59 m by 2100.

The above aspects are all related to temperature changes in the Earth’s atmosphere or oceans. Such warming is but one aspect of climate change. It is also expected that changes will occur in precipitation which in turn will affect other weather variables such as levels of humidity.

Of greater interest and importance at the local scale are the expected changes in the frequency or intensity of extreme events. 

Prediction of extreme events is a difficult task, since by definition they are rare (thus extreme) and they often involve the simultaneous combination of a number of different factors occurring in critical locations or at critical threshold levels.  Furthermore, extreme events have occurred throughout history and it is therefore difficult to directly correlated currently observed events with climate change.  However there has appeared to be an increased propensity for extreme events to occur around the globe over recent decades.  Some of the most catastrophic weather- and climate-related events of recent years have included:

•  The European heatwave in the summer of 2003, during which tens of thousands of deaths occurred, mostly involving elderly populations in France.

• Hurricane Katrina, which caused flooding which devastated the city of New Orleans as levees breached.  Over 1,000 deaths occurred and more than 80% of the city was inundated.

•  Australian and southern European bushfires, which have now become more unpredictable due to their increased frequency and volatility.

•  The Amazonian drought in 2005, during which many rivers ran dry through this fragile ecosystem.

•  The more extreme and elongated typhoon season in China of 2003, during which extensive flooding and landslides occurred.

•  The monsoon in Mumbai, India in June 2005, which resulted in over a thousand deaths due to flooding.

•  The collapse of snow-laden building roofs in southern Germany and Austria in the winter of 2005/06.

The summer of 2003 was an unusually warm and not just in the continent of Europe, and temperature records were broken in many places.  There were also positive temperature anomalies of three degrees or more compared to the late 19th Century over large parts of Europe in the August of that year. 

The warm summer caused great losses in agricultural productivity in southern Europe, and during the hottest days there were around 20,000 deaths in urban areas such as Paris. Using a combination of observations and modelling, recent research from the Met Office Hadley Centre and Oxford University estimates with high probability that the risk of such anomalously high European temperatures has already doubled due to the effects of human-induced climate change.  This research also concluded that the 2003 European heatwave was partly due to human-induced climate warming and that later in the 21st Century such summers may occur, on average, every other year (Stott et al. 2004).

The climate change that may occur towards the end of the century is very much dependent on our present day and near-future global emissions. These are aspects that can be controlled now, but to do so requires a global commitment in the face of some severe challenges.  For example, most emissions to date have been the result of industrialisation in the western world. China, with a population of 1.3 billion people, is heading for an energy-intensive lifestyle and over the next 20 years is predicted to be the world’s largest emitter of greenhouse gases.  This is an indication of the scale of the challenge facing those tackling climate change across the world: failure to decrease our current global emissions and rate of growth may ultimately push the Earth’s climate system to a critical point where change is no longer gradual and progressive, but instead becomes sudden and extreme.

Changes in the UK Climate

The rates of global climate change that have been observed over recent historic timescales are unprecedented. The consequences of these global climate changes are increasingly being felt across the UK with impacts occurring which are projected to intensify according to findings from UKCIP and the Met Office’s Hadley Centre.

The 1990s was the warmest decade in central England since records began in the 1660s and this warming of climate over land has been accompanied by warming of UK coastal waters, as exemplified by the red and blue lines respectively on this figure. (source: UKCIP)

In the summer of 2003, when a heatwave affected many part of Europe, a record high temperature of 38.5ºC was recorded for the UK at Faversham in Kent. This was 1.4ºC higher than the previous record set in 1990 at Cheltenham in Gloucester.

Other observed indicators of notable UK climate changes include:

• changes in the timing of natural events (such as flowers and tress coming into leaf earlier),
• the growing season for plants in central England lengthening by about one month since 1900;
• heatwaves becoming more frequent in summer;
• fewer frosts and winter cold spells;
• winters over the last 200 years having become much wetter relative to summers throughout the UK;
• larger proportion of winter precipitation (rain and snow) now falling on heavy rainfall days than was the case 50 years ago; and
• the average sea level around the UK now being about 10cm higher than it was in 1900.

Studies of the observed distribution of precipitation events in the UK over recent decades found that in winter there has been a decline in light and medium precipitation events and an increase in the heaviest events, relative to the total amount of winter precipitation. In the summer the reverse was true, i.e. there has been a decline in the proportion of the seasonal total being provided by the heaviest events.

‘Looking Towards the Future Climate in the UK’ the UKCIP02 climate scenarios project states that average temperatures may rise from between 2ºC and 3.5ºC by the 2080s, with the temperature of coastal waters also increasing. The rate of annual precipitation may decrease slightly though there will be a change in the seasonal distribution of this rainfall with summer becoming drier and winters wetter. Spring temperatures may occur up to three weeks earlier with winter temperatures delayed by the same amount. Extreme summer temperatures are likely to become more frequent and will, in all likelihood, coincide with dry conditions more regularly.  Very cold winters are predicted to become less common, extreme winter precipitation more frequent and snowfall rates will decrease significantly. 

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