The Earth´s climate is changing. The world´s climate is in constant change with severe weather events occurring all the time which means it is difficult to attribute any one extreme event to human induced global warming. However humanity is now considered responsible for most of the warming of the past 50 years*15 and despite natural variability there is now evidence of more extreme events such as heavier and more intense rainfall as well as more frequent heatwaves. Europe’s heatwave of 2003 lay so far outside the range of natural variability that human influence is believed to have contributed to its severity.*16

As temperatures continue to increase, chaotic and unpredictable impacts could cascade across the globe causing potentially rapid and irreversible changes.

Precipitation

A warmer world causes heaver and more frequent rainfall. Rising temperatures increase evaporation from Earth´s surface whilst at the same time for every 1C rise in temperature the atmosphere can hold about 7% more water.*17 The combined effect is a more intense water cycle. Already precipitation in some regions of northern Europe has increased by 10 to 40% during the 20th century.*18 and in the United States very heavy precipitation has increased by 20% largely since the 1970´s.*19 As temperatures rise this trend is likely to continue.

Drought

In other regions rainfall has decreased. Additionally hotter temperatures cause land areas to dry out more. The combined effect is that dry areas across the world have more than doubled since the 1970s,*20 with more intense and longer droughts occurring over larger areas, particularly in the tropics and sub-tropics.*21 By 2050 annual average river runoff and water availability could decrease by 10 to 30% in some dry land areas and in the dry tropics, some of which are already suffering water shortages.*22 And with high emissions the proportion of the Earth’s total land area suffering extreme drought is expected to increase dramatically from 1% in the 20th century to up to 30% by 2100.*23

Hot and Cold Days and Nights

In the last 50 years, cold days, cold nights and frost have become less frequent, while hot days, hot nights and heatwaves have increased across the world.*24 In the UK, twice as many hot summer days now exceed 25°C than during the first half of the 20th century, resulting in more heatwaves.*25 By 2050 European heatwaves could be 4° to 10°C hotter*26 and much more frequent which will cause more heat-related fatalities.

The question is : How hot could it get. And how much heat can humanity take ? Already some regions of the world have seen temperatures rise to 50C.

Winds

Global wind patterns and the strength of winds are being affected as temperatures rise.*27 In the southwest Pacific Ocean the most powerful hurricanes – category 4 and 5 – have more than doubled in number since the 1970s.*28 In the North Atlantic and western North Pacific the power released by hurricanes has increased by about 75%, partly due to warmer sea surface temperatures (SSTs).*29 Hurricanes and typhoons only develop when SSTs exceed about 26°C, so a warmer world is likely to extend the range of such storms and increase their intensity.*30

The Melting Cryosphere

The Earth’s ice cover, the cryosphere, is melting. Glaciers and ice caps are receding, ice shelves collapsing, snow cover decreasing, sea ice melting and by 2020 Mount Kilimanjaro’s ice cap could disappear for the first time in 11,000 years.*31 And as the ice on our planet melts, light white surfaces of ice and snow, which reflect up to 90% of the sun’s rays back into space, are replaced by darker ocean or land surfaces, which reflect only about 10%.*32 This means more of the sun´s heat will be absorbed by the Earth´s surfaces increasing the warming and melting in a dangerous cycle that accelerates global warming.

Arctic sea ice

Arctic temperatures at the north pole are rising about 0.5°C every decade.*33 As a result the Arctic Ocean’s winter ice cover has decreased from around 16 million km2 in the 1960s to only 14 million km2 by 2006.*34 In summer 8 million km2 of sea ice used to remain frozen,*34 but is now experiencing repeated and extensive ice melt. In 2007, the worst year on record, almost 50% of the former summer sea ice expanse melted.*35 Additionally at its centre Arctic sea ice is now 40% thinner than in the 1970s.*36 By 2100 up to 50% of the Arctic’s average sea ice cover could disappear while late summer sea ice may be lost almost completely even sooner.*37

Greenland´s Land Ice

More of the surface of the Greenland ice sheet is melting in summer due to higher temperatures.

This is now contributing to sea level rise.*38 125,000 years ago when polar temperatures were 3° to 5°C warmer, much of the ice had melted and contributed to raising sea level which was 4 to 6 metres higher than today.*39 Yet Arctic annual average temperatures could rise by 3.3 to 8.3°C above the 1850-1899 average this century - to a similar or even higher level than 125,000 years ago*40 which will cause significant ice loss.

If the world´s average temperature rises in excess of 1.9 to 4.6C and this is sustained Greenland´s ice sheet will melt almost completely over the next thousands years raising sea level about 7 metres.*41

Antarctic

The Antarctic Peninsula, at the south pole, has warmed rapidly with temperatures in some regions up to 2.5C warmer than fifty years ago.*42 Five massive ice shelves broke up between 1995 and 2002*43 then in 2008 part of the Wilkins Ice Shelf collapsed,*44 with the bridge connecting the ice shelf to two islands also breaking apart in April 2009.*45 Since the 1950s, 90% of the Antarctic Peninsula´s 244 glaciers have also been melting.*46 Parts of the West Antarctic Ice Sheet (WAIS) are also melting unexpectedly. If it were to melt completely it would contribute 5 metres to sea level rise over the next thousand years or more.*47

Sea Level rise

Over 80% of the heat added to the climate system has been absorbed by the global oceans, which have warmed to depths of at least 3000 metres.*48 Warmer oceans cause sea water to expand and contribute to raising sea level. During the 20th century global average sea level rose 17 cm, at an annual average rate of 1.8 mm between 1961-2003. Between 1993-2003 this increased to 3.1mm a year.*49 Rising sea level is eroding coastlines, inundating coastal wetlands, causing salt water to invade fresh water supplies. Land is also being lost to the sea. By 2100 average global sea level could increase another 18 to 59 cm if Greenland and Antarctica´s contribution stays at 1993-2003 melt rates. If it grows linearly as the world´s temperature increases sea level could rise up to 79cm this century.*50

Ocean currents

As mountain glaciers and ice sheets melt and as precipitation patterns change, more fresh water flows into the sea, reducing its salinity or salt content. This could slow the Meridional Overturning Circulation (MOC). Ocean currents transport heat from the equator to the polar regions, where dense with salt they sink and flow back along the bottom of the sea towards the equator. By 2100 the MOC could slow by 0 to 50%.*51

Warming acidic Oceans

The oceans have been absorbing about two billion tonnes of the carbon produced by human activities every year, which is making them more acidic.*52 The extra acidity is likely to affect corals and other shell-forming organisms from growing their calcium carbonate shells and could damage some types of plankton at the base of the food chain. Coral and plankton numbers could decrease and would reduce the fish populations that feed on them.*53 Additionally the warmer oceans could affect the oceans ability to continue to absorb carbon dioxide. This would leave more in the atmosphere and so increase global warming.*54

The Natural World

Up to one third of the Earth’s plant and animal species face extinction if the world’s average temperature rises a further 1.5°C *55 Even a 0.5° to 1°C of warming per century is too fast for many species to adapt to as their environment changes.*56

• Currently 35% of the world’s bird population, 50% of amphibians and 70% of warm-water corals are endangered.*57


• By 2050 two thirds of the polar bear population could be lost.*58


• With a further 1.5° to 3°C rise in the Earth´s average temperature coral reefs are likely to be wiped out globally.*59


• At least 20% to 40% of the Amazon is expected to be committed to die-back with even a small warming.


• With a 4-5°C increase in the world´s average temperature about 90% of the forest could be gone and all the species that inhabit it.*60


• With a 3°C rise 20% to 50% of plant and animal species face extinction.*61

Once lost they are gone forever.