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Carbon Brief's guide to the IPCC report

  • 27 Sep 2013, 13:45
  • Freya Roberts

 

As global greenhouse gas emissions continue to rise, the earth is warming, snow and ice is melting, and sea levels are rising. These are the confident conclusions from a brand new report from the Intergovernmental Panel on Climate Change (IPCC). 

 

The full report runs to several thousand pages, but scientists have distilled its contents into a 36-page summary - intended to capture the findings most relevant to policymakers. Here are the bits you need to know about:

Temperatures

One of the clearest signs of climate change is rising temperatures. Between 1880 and 2012, earth's surface warmed by approximately 0.85°C, and the first decade of the 21st century was the hottest since modern records began in 1850. Scientists are 95 per cent certain humans' influence on the climate is the dominant reason earth warmed between 1951 and 2010.

The report notes that within the long term warming trend, short periods of slower surface warming have occurred. Between 1998 and 2012, for example, earth's surface has warmed at a rate of 0.05°C per decade - which is slower than the trend since 1951 of 0.12°C per decade. The report notes:

"Due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends."

The current slower rate of warming is due to two factors - roughly in equal measure - says the report. First, natural fluctuations in the climate system have cooled surface temperatures, to some extent by redistributing heat within the ocean. 1998 was an unusually warm year due to an ocean and atmosphere cycle known as an El Niño event. And second, the amount of sunlight reaching earth's surface has declined - partly due to natural cycles in the sun's orbit and partly due to the release of volcanic ash which reflects incoming sunlight.

Despite the short slowdown, the report predicts the long term trend of warming will continue. As carbon dioxide levels in the atmosphere double, earth is expected to warm by between 1.5 degrees Celsius (°C) and 4.5°C. By 2100, that could translate to as little as 0.3°C warming compared with 1986-2005 levels - but only if we undertake drastic emissions cuts from 2020. Following a high emissions pathway means temperatures could rise by 4.8°C.

Oceans

The oceans are another indicator of major changes in the earth's climate. By absorbing human-produced carbon dioxide,  the oceans are becoming more acidic. On top of that, they are warming. The oceans have taken up more than 90 per cent of the heat trapped by greenhouse gases since the 1970s, the report states.

Water expands as it warms. At the moment this thermal expansion of water is the main reason sea levels are rising. Over the last two decades (1993-2010), they've risen by more than three millimetres per year, but much more is in store in the future, the report says.

With radical emissions cuts, sea level rise may be limited to 26 centimetres by the end of the century. If not, up to 82 centimetres of sea level rise is possible. That's a big increase on the predictions in the IPCC's last report of between 18 and 59 centimetres. Since then, scientists knowledge of ice sheets have improved, allowing them to include ice sheet melt in sea level projections.

Ice sheets, glaciers and sea ice

Both Antarctica and Greenland are losing mass, the report says. With very few exceptions, glaciers around the world are melting too. The melting of the glaciers and ice sheets, which the report says is very likely to have been influenced by human activities, is the other major factor driving up sea levels.

Rising temperatures are shrinking and thinning sea ice too. In the Arctic, the area of ocean covered with ice has been shrinking in every season, and every successive decade since satellite records began in 1979. The report notes that the loss of Arctic sea over the past three decades, at a rate of around 3.5 to 4.1 percent, is unprecedented. The change in summer months has been particularly strong, with ice extent decreasing 13 percent per decade.

Sea ice surrounding the Antarctic ice sheet has not behaved in the same way as Arctic sea ice, growing by between 1.2 and 1.8 per cent per decade. Scientists are still uncertain about why this has happened. There is little data collected about the vast region, which makes it difficult to separate natural from human-caused changes.

Extreme events

Climate change is also manifesting as changes to extreme weather events. For some extreme events, the changes are clear. For example, it's very likely there are already fewer cold days and more hot days around the world - a trend the report states is virtually certain to continue in the future. Large parts of Europe, Asia and Australia have also experienced more frequent and long lasting heatwaves - another trend that's very likely to continue looking forward.

For other extreme weather there isn't yet enough data for scientists to be confident about future and past trends. For both droughts and hurricanes, it's less clear whether there have been changes in past trends and to what extent humans' activities have contributed. Despite this, scientists are still able to identify that certain parts of the world have experienced more drought, and that in the North Atlantic for example, the intensity of hurricanes has increased.

Looking forward, it's more likely than not - so more than a 50 per cent chance - that hurricanes will become more intense in the Western North Pacific and North Atlantic. It's also likely drought will last longer and become more intense on a "regional to global" scale.

Looking to the future

As the report shows, major changes to the climate system are already taking place, and are set to continue to the end of the century and beyond. But the report also spells out that the severity of these changes depends on the emissions pathway humans choose.

For the first time, the IPCC has laid out a 'budget' for human emissions, starting from the beginning of the industrial era, that we will have to keep within to limit temperature rise to widely-considered safe target of two degrees.

To have a 66 per cent chance of staying within that budget, humans can only emit 1000 billion tonnes of carbon, or around 800 billion tonnes when you include other factors that warm the climate system. By 2011, more than 500 billion tonnes of carbon had already been released.

The IPCC's lowest emissions scenario suggests keeping below two degrees is possible if global emissions peak by 2020 and decline rapidly after - something that's unlikely unless there's an international emissions agreement. If not, we're in for much greater warming, faster sea level rise, and a new era of extreme events.

 

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