The Intergovernmental Panel on Climate Change (IPCC) is the leading international body for the scientific assessment of climate change known mostly for its comprehensive Assessment Reports, published about every six years since 1990.
The 5th Assessment Report (5AR) will be published during 2013 and 2014 in four stages. The first release - Working Group 1 report (WG1) – was launched on September 27th, 2013 in Stockholm.
The WG 1 report summarises what we know about climate change, what is causing it, how it can be observed and what’s ahead in terms of temperature increase, sea-level rise, glacier melting, extreme weather events and so on, depending on future emissions.
The WG1 report does not discuss in detail the impacts of warming or ways to prevent it, so what have we learned?
1) Human-caused climate change can be detected around the world.
Greenhouse gas emissions have warmed the atmosphere and oceans, melted glaciers, raised sea levels, changed water cycles and increased some extreme weather events. The process has only just started but the report suggests that it is now virtually certain that human influence has changed the climate.
2) The decade of the 2000s has been the warmest in the instrumental record.
Global-mean surface temperatures have risen at a slower pace somewhat slower in the past 15 years but overall warming continues and the climate system as a whole, including the deep ocean, has continued to accumulate energy.
Observations of CO2 concentrations, average temperature and sea level rise are generally well within the range of the earlier IPCC projections.
There are alarming signs of accelerating impacts. In the past decade (2002-2011) the Greenland Ice Sheet was losing mass about six times faster on average than just the decade before. The Antarctic ice sheet was losing mass five times faster.
Since 1993 sea-levels have risen twice as fast as in the past century on average. Sea ice extent in the Arctic has been diminishing significantly faster than projected. The North Pole, a place we’ve learned to think of as permanently covered by ice, could be ice free in the summer months.
3) Impacts will worsen significantly with increased emissions
There are many impacts we can no longer avoid, even if emissions are cut fast. Temperatures will continue to increase, ice to melt, sea-level to rise, permafrost to retreat and extreme weather events will increase.
The IPCC has assessed four new scenarios for the future, one that would keep warming below 2 degrees Celsius (with median warming of about 1.5°C by 2100), one that’s “business as usual”, and could result in close to 5°C warming by 2100, and two in between.
4) Emissions must peak and start declining before 2020
There’s not much “space” left in the atmosphere for further emissions, if we want to avoid the worst impacts. If our fossil fuel emissions continued to grow by 3.2 % a year, as they did in 2000-2009, almost half of our remaining “budget” would be used up in just little over a decade.
Following the lowest scenario assessed by the IPCC, that gives a 66% likelihood of staying below 2°C, growth of global emissions would need to peak before 2020 and rapidly decline towards zero emissions by 2070. Aiming for higher certainty, ruling out negative emissions or getting a slower start will require faster emission cuts.
5) The long-term warming trend is robust.
Over shorter periods the rate of warming can vary and either be somewhat faster or slower. Trends on short periods like 15 years can vary a lot and do not in general reflect long-term climate trends. About half of the slower warming since 1998 can be attributed to natural variation especially the interaction between the atmosphere and the ocean.
Put simply the report suggests that a larger part of the warming has been taken up by the ocean and less by the atmosphere (medium confidence).
The other half, the IPCC concludes, can be attributed to current phase of the sun’s 11-year cycle and the action of a series of small volcanic eruptions. This temporary slow-down of warming doesn’t change the nature of long-term warming trend.
