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Tim Dodd

28.11.2012 | 3:00pm
ScienceFuture climate projections: Five graphs from three reports
SCIENCE | November 28. 2012. 15:00
Future climate projections: Five graphs from three reports

Three different reports released in advance of the latest instalment of the  international climate talks in Doha all carry more or less the same message. On current trends the world is increasingly unlikely to avoid a temperature rise of two degrees above pre-industrial levels – and a four degree rise is possible, they say.

We’ve pulled out some of the most interesting statistics and graphs to summarise the different reports.

The World Meteorological Organisation’s greenhouse gas bulletin

First, the World Meteorological Organisation (WMO) – a specialised United Nations agency – released its annual greenhouse gas bulletin. The findings are based on data gathered by the WMO’s Global Atmosphere Watch Network, which monitors the amount of greenhouse gases in the atmosphere.

The WMO’s top-line conclusion is that greenhouse gas concentrations in 2011 were the highest on record. Carbon dioxide, methane and nitrous oxide – the greenhouse gases driving additional warming – are now 140 per cent, 259 per cent and 120 per cent higher than pre-industrial levels, respectively.

Given that global greenhouse gas emissions continue to accumulate in the atmosphere, the ‘record high’ may not be that surprising. But the WMO also concludes that despite international commitments to cut greenhouse gas emissions, the amount of carbon dioxide in the atmosphere has continued to increase at a fairly steady rate over the last decade.

The report also looks briefly at the role of forests and oceans as carbon sinks, which absorb some manmade emissions. The graph below shows that as carbon emissions have increased (yearly change, in green), the concentration of carbon dioxide in the atmosphere has also increased (yearly change, in blue). But not as fast as it might have done – because around half of carbon dioxide emissions have been absorbed by the world’s carbon sinks (yearly change, in red):

Screen Shot 2012-11-22 At 12.34.23

Source: WMO Bulletin. The graph shows annual emissions of carbon, annual increase in concentration of carbon, and the amount of carbon sequestered by sinks each year. The figures are based on research by Ballantyne and colleagues, 2012 and Levin, 2012.

In other words, the natural world has absorbed some of the rise in manmade carbon emissions, although it may not continue to perform this service for us in the long term.

UNEP emissions gap report

The United Nations Environment Programme (UNEP)’s emissions gap report is compiled on an annual basis. It aims to calculate the ‘gap’ between emissions reductions promised by governments around the world, and the emissions cuts needed to limit temperature rise to two degrees Celsius.

There’s a range of uncertainty in projecting future temperature rise, but the gap is getting bigger, says UNEP. It’s illustrated in the following infographic, which also captures the range of uncertainty in the two degrees projection – (in grey):

Screen Shot 2012-11-22 At 13.04.40

The grey descending patch shows the range of emissions reductions from all greenhouse gases consistent with being on a track for a two-degree temperature rise. (The fact that this is a range, rather than a single pathway, reflects uncertainty.)

The four green lines show different projections of global emissions pathways, based on promises made by countries at the moment and the extent to which they are implemented. UNEP’s conclusion – reflected in the graphic – is that there is still a substantial gap between political ambition and limiting temperature rise to two degrees Celsius.

Future emissions by country

Less abstractly, the UNEP report also includes interesting graphs of past, present and (possible) future emissions from different parts of the world.

This graph shows emissions from a selection of G20 countries since 1990, and information on what those countries have said their emissions will be by 2020:

Screen Shot 2012-11-27 At 12.33.41

Source: Figure 2.6, UN Emissions gap report. The graph uses national-level data.

China’s projected emissions growth sticks out. To add detail, the next chart changes the pattern considerably by comparing emissions per capita, (per person) rather than overall emissions. At current rates of growth, China’s per capita emissions are going to remain substantially below those of some other developed countries – although according to this assessment they will reach or surpass European levels.

Screen Shot 2012-11-27 At 12.35.20

Source: Figure 2.8, UN Emissions gap report. The graph uses UN population data.

World Bank – Four degrees

The World Bank also released a report at the beginning of last week, warning that if countries don’t fulfil current emissions reduction promises, the world could see four degrees celsius temperature rise by the end of this century.

Projecting future temperature rise is tricky, as it depends on how sensitive the climate is to carbon emissions in the longer term, and how man made emissions change over the century.

But even with such uncertainty in the system, the message of the report is that the current level of ambition on emissions cuts in not adequate to rule out higher levels of temperature rise.

The graph below illustrates temperature projections for a selection of future emissions scenarios – and the one for current pledges to cut emissions (in purple) exceeds the limit of emissions which gives a fifty per cent chance of avoiding more than two degrees celsius temperature rise.

Screen Shot 2012-11-27 At 14.14.16Source: Figure 22, ” Turn down the heat“, World Bank. The high emission scenarios are created by the IPCC. Data for the low-emission scenarios taken from Hare and colleagues, 2011, Rogelj and colleagues 2010 and Schaeffer and colleagues 2012.

The “current pledges” scenario – the purple line – gives a 50/50 chance of a temperature rise above 3 degrees celsius, the report says. But it also notes:

“Even with the current mitigation commitments and pledges fully implemented, there is roughly a 20 percent likelihood of exceeding 4°C by 2100.”

Three reports – roughly the same message

Despite promises from countries to cut their emissions, the world’s greenhouse gas emissions continue to rise, making higher levels of temperature rise this century more likely.

That’s the clear conclusion of these three reports, which also suggest that the task of limiting temperature rise to 2 degrees or less above pre-industrial levels is becoming increasingly challenging. In contrast, future scenarios where the world experiences a temperature rise of 4 degrees or higher are looking increasingly plausible.

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