The world has received the clearest message yet on how humans are changing the climate. Delegates from 195 countries gathered in Copenhagen this week to add their seal of approval to a 100-page “synthesis report”. It’s the final instalment in a four-part series from the UN’s Intergovernmental Panel on Climate Change (IPCC).
The synthesis report condenses the IPCC’s three other major reports on different aspects of climate change into one concise document. While that means some parts of it may sound familiar, there are some new and different sections as well. Here’s our assessment of what’s new, as well as a look at the report’s main conclusions.
Warming continues unabated
Evidence that the climate is warming is unequivocal, the IPCC says. Each of the last three decades has been warmer than any previous one since 1850, and the world has warmed by about 0.85 degrees since then.
Today’s report explains how the rate of surface warming varies from decade to decade, noting that warming since 1998 has been a third to half of the average since the 1950s. But, it adds:
“Even with this reduction in surface warming trend, the climate system has very likely continued to accumulate heat since 1998”.
A new graphic for today’s synthesis report, showing surface temperature change, sea level rise, global greenhouse gas concentrations and carbon dioxide emissions from human activity since 1850. Source: IPCC 5th Assessment Synthesis Report
A stronger message on human influence
On the fundamentals of climate change, the report concludes that greenhouse gas emissions from human activity between 2000 and 2010 were the highest in history, contributing to levels in the atmosphere unprecedented in at least 800,000 years.
Since 1970, total carbon dioxide emissions from fossil fuels and cement production have tripled while emissions from forestry and other land use have risen by about 40 per cent.
A key finding from the IPCC last year was that it’s extremely likely humans are responsible for more than half of the warming since the 1950s. The chart below from today’s synthesis report shows scientists’ best guess is that human activity can account for all the warming we’ve seen. The orange bar, showing the human contribution to warming, is about the same size as the black bar, which is the warming we’ve seen since 1951.
Source: IPCC 5th Assessment Synthesis Report
We’d likely have seen even more greenhouse gas warming (green bar) if it weren’t for other particles, released as fossil fuels burn, that have an overall cooling effect (yellow bar).
This figure featured in one of the underlying chapters from the first IPCC report, but it’s been reworked and given more prominence in the Summary for Policymakers – a distilled 40-page version of today’s Synthesis report containing what are seen as just the most policy relevant findings.
Acidifying oceans, sea level rise and ice melt
About a third of the carbon dioxide put into the atmosphere goes into the oceans, causing them to become more acidic, the report notes. The current rate of ocean acidification – an increase of 26 per cent over the industrial period – is unprecedented in the past 65 million years, the report notes.
The oceans are heating up too. The top 75 metres are warming fastest but heat is penetrating deeper down, warming water as far down as 2,000 metres.
Globally, sea levels have risen by 19 cm since 1901, and are accelerating. If emissions continue to rise at current rates, we can expect a further 45 to 82 cm metres by the end of the century, exposing 70 per cent of the world’s coastlines to higher seas and greater flood risk.
Today’s report also discusses the increase in sea ice surrounding Antarctica, but is clear that global ice cover is in decline. Glaciers are shrinking and Arctic sea ice has decreased in every season and every decade since 1979, currently at a rate of about four per cent per decade.
A new carbon budget
For the first time in IPCC history, last year’s report calculated the remaining amount of carbon humans can emit and still have a likely chance of limiting warming to less than two degrees above pre-industrial temperatures – the UNFCCC internationally accepted goal.
The total budget is 2,900 billion tonnes of carbon dioxide. From the industrial revolution through to 2011, we’d emitted 1,900 billion, meaning two thirds of the budget was already spent. Another three years’ of emissions, which are currently around 38 billion tonnes per year, means the remaining budget is even tighter.
But while the international community considers two degrees as the threshold for dangerous warming, many low-lying and island nations that are already seeing the effects of sea level rise warn that for them, even 1.5 degrees of warming comes with unacceptably high risks.
Another new feature of today’s report is the calculation of a carbon allowance to stay below 1.5 degrees. The existing budget for two degrees already requires that about 80 per cent of known fossil fuel reserves stay in the ground, and keeping below 1.5 degrees is more ambitious still.
Consequences of inaction
Climate change isn’t a dim and distant prospect, the report suggests. Some places around the world are already feeling the impacts. And the risks get higher with greater levels of warming.
Climate change is causing more heat waves and heavy rainfall in a number of regions, increasing flood risk and compromising food and water security, among other impacts.
Tackling climate change requires collective action on a global scale, the report says. A likely chance of limiting warming to two degrees means cutting carbon dioxide emissions by 40 to 70 per cent by 2050, with unabated fossil fuel burning almost entirely phased out by 2100.
Simultaneously, low or zero carbon energy sources, including wind, solar and nuclear, will need to increase from the current share of 30 per cent to more than 80 per cent by 2050. Renewable technologies have increased in maturity in recent years, while the costs have come down.
The report talks about the possibility of temporarily overshooting the target and removing carbon dioxide from the air. But this relies on a technology known as Bioenergy with Carbon Capture and Storage (BECCS), the potential of which is untested and highly uncertain.
Counting the costs
A low-carbon transition will require big changes in investment patterns but is affordable, the report suggests. Economic modelling suggest that limiting warming to below two degrees through the 21st century would shave about 0.04 to 0.14 per cent off annual growth.
But that’s compared to a hypothetical world where we feel no impacts from climate change and economic growth continues unhampered.
The savings from avoiding climate impacts together with co-benefits like improved air quality should mean real-world costs are lower, and should be weighed against the cost of doing nothing.
Even with mitigation, past emissions mean some climate change impacts are unavoidable. Increasing the resilience of human society through adaptation will help bring those risks down. But there’s no single solution, and the approach will be different between regions, communities and ecosystems.
Delaying action shifts the burden from present to future generations and will make reducing climate risks harder and more expensive to achieve, the report notes.
What happens now?
Today’s report isn’t a rulebook. It’s designed to act as a guide for policymakers on how to avoid the most serious climate change risks, should we collectively decide that’s worth doing. Where we go from here rests largely on what level of risk we’re willing to expose ourselves to.
As today’s report says:
“Decision making about climate change is influenced by how individuals and organizations perceive risks and uncertainties and take them into account.”
However, the report does clearly and concisely lay out scientists’ best understanding of the science of climate change, in a document that will be closely scrutinised by policymakers. However the politics of climate progress, there can be little doubt that a comprehensive assessment of the science on climate change is now firmly on the table.
The IPCC, in one infographic