Understanding climate change — SOER 2010 thematic assessment

• Key facts
• Key messages

Summary

What are the current and projected future greenhouse gas concentrations?

In the year 2009, the atmospheric CO₂ concentration was about 387 ppm, which is 38 % above the pre-industrial level of 278 ppm. The concentration of the six greenhouse gases (¹) covered by the Kyoto Protocol reached 438 ppm CO₂-equivalent in 2008, an increase of 160 ppm from the pre-industrial level. Under the emissions scenarios of the Intergovernmental Panel on Climate Change (IPCC), the overall concentration of the six 'Kyoto gases' is projected to increase to 638–1 360 ppm CO₂-equivalent by 2100.

What are the main changes in the climate system?

The global mean temperature in 2009 was between 0.7 and 0.8 °C higher than in pre-industrial times and the decade 2000–2009 was the warmest on record. Europe has warmed more than the global average. The annual average temperature for the European land area was 1.3 °C above the 1850–1899 average. Without global emission reductions, the IPCC expects global temperatures to increase further by 1.8–4.0 °C above 1980–1999 levels by 2100. Global temperature increase would exceed 2 °C above industrial times – the limit agreed by the EU — between 2040 and 2060 in all IPCC scenarios. The rise in temperatures has had, and will continue to have, serious impacts on various parts of the climate system. Some examples are:

• The extent of Arctic summer sea ice has declined by about 10 % per decade since 1979. The extent of the minimum ice cover in September 2007 was half the size of the normal minimum extent in the 1950s; the third lowest minimum occurred in September 2010. Summer ice is also getting thinner and younger.
• Observed global mean sea-level rise has accelerated over the past 15 years. From 2002 to 2009, the contributions of the Greenland and West Antarctic ice sheets to sea-level rise increased. In 2007, the IPCC projected a sea level rise of 0.18 to 0.59 m above the 1990 level by 2100. Recent projections show a maximum increase of about 1.0 m by 2100, while higher values up to 2.0 m cannot be excluded.
• Glaciers in the Alps lost about two-thirds of their volume between 1850 and 2009. The glacierised area in the Alps is projected to decrease to about one-third of the present area with a further rise in Alpine summer temperature of 2 °C.
• Acidification is occurring in all ocean surface waters as a result of increased atmospheric CO₂ concentrations. Coral reefs worldwide, which are centres of biodiversity and important as fish breeding grounds, are threatened by both ocean acidification and increasing temperatures. By 2100 the pH value could drop to 7.8, an increase in ocean acidity by 150 % compared to the pre-industrial pH of 8.2. The acidity of the ocean would be higher than at any time in the past 20 million years.

What are the global risks of climate change?

Land and ocean sinks have taken up more than half of global CO₂ emissions since 1800. But these natural sinks are vulnerable. They are highly likely to take up less CO₂ in the future. Moreover, poor nations and communities, ecosystem services and biodiversity are particularly at risk. A temperature rise of more than 1.5–2 °C above pre-industrial levels could cause disruptions in many regions. Unabated greenhouse gas emissions increase the risk of large-scale irreversible shifts in the climate system with potentially serious consequences for society and ecosystems. Recent research suggests that several key components of the climate system could undergo irreversible change at significantly lower levels of global temperature increase than previously assessed. The most important 'tipping elements' for Europe are the Greenland ice sheet, Alpine glaciers and Arctic sea ice.

What are the targets to limit global climate change?

To limit impacts and guide policy development, the Copenhagen Accord of December 2009 recognised a long-term climate limit of 2 °C global mean temperature increase, without specifying the base year. The Accord also mentions the need for a review in 2015 to consider a possible goal of limiting temperature rise to 1.5 °C on the basis of new scientific insights. According to the IPCC (2007), confirmed by later scientific insights, to have a 50 % chance of limiting the global mean temperature increase to 2 °C above pre-industrial levels, the atmospheric greenhouse gas concentration needs to be stabilised at about 445 to 490 ppm CO₂-equivalent (or about 350 to 400 ppm CO₂). To achieve this, global emissions should peak at the latest in 2015–2020 and decline to 50–80 % below 2000 levels by 2050.

(¹) Carbon dioxide (CO₂), methane (CH₄), nitrous oxide which is also known as laughing gas (N₂O), hydrofluorocarbons (HFC), perfluorocarbons (PFC) and sulphur hexafluoride (SF₆). Greenhouse gases are often measured in CO₂-equivalent in order to allow for comparisons of their potential to contribute to global warming.