Global warming unequivocal

Photo: Flickr.com/NASA Goddard Photo and Video/CC BY

The first IPPC carbon budget to limit global warming to 2 degrees will be exceeded in three decades with current rate of carbon dioxide emissions.

After a four-day meeting in Stockholm, on 27 September, the Intergovernmental Panel on Climate Change (IPCC) Working Group I presented its long-awaited contribution to the fifth assessment report on Climate Change. The over 1,000-page report by 259 lead authors has been reviewed by more than one thousand experts, who made 54,677 comments.

In the fourth report from 2007 the IPCC concluded that it is “very likely” that human emissions of greenhouse gases, rather than natural variations, are warming the planet’s surface. In this report, the wording has intensified to “extremely likely”. (Box) Warming of the climate system is described as “unequivocal”.  Each of the past three decades has “very likely” been warmer than any of the preceding decades since 1850. However global warming is also observed over much longer time periods. In the northern hemisphere, the period 1983–2012 was “likely” the warmest 30-year period of the last 1,400 years. This time period includes the medieval climate anomaly (AD 950–1250), a warm period sometimes highlighted by climate sceptics as evidence against human-induced global warming.

Levels of carbon dioxide, methane and nitrous oxide are “unprecedented” in at least 800,000 years. Concentrations of carbon dioxide have increased by 40 per cent since pre-industrial times, most of it originating from fossil fuels, but also from land-use change. Oceans have absorbed about 30 per cent of the emitted carbon dioxide, a process that has caused ocean acidification.

For the first time the IPCC presents figures for how much CO₂ can be emitted to give different probabilities of limiting warming to less than 2 degrees compared to 1850 (table). Of the total carbon budget for the higher degree of probability (66%) (3,670 GtCO₂) more than half (1,890 GtCO₂) was already emitted by 2011.

Table: Remaining scope for CO₂-emissions according to the IPCC for different probabilities if warming is to be limited to 2 degrees compared to 1850. As the impact of non-CO₂ forcing can vary between different scenarios, the IPCC budget has been based on a calculation that only takes into account warming caused by carbon dioxide and assumes non-CO₂ forcing according RCP2.6. However it is unlikely that all of the non-CO₂ forcing could be eliminated, so the first figure should be considered more as a theoretical maximum. By dividing the remaining carbon budget by the 2012 annual emission level of 35 GtCO₂, we get an idea of how much time is left before the margin is used up, provided that future emissions remain at the 2012 level.

Taking into account other anthropogenic non-CO₂ forcers such as nitrous oxide, methane and particles the space left for future emissions is even smaller. Assuming that current emission levels remain constant (they are actually increasing), there is less than 30 years before the entire emissions budget is exhausted. For a higher degree of probability or a lower temperature target, time is even scarcer.

In its press release the IPCC calls for “substantial and sustained reductions of greenhouse gas emissions” to limit climate change.

The IPCC also presents a set of new emission scenarios, known as Representative Concentration Pathways (RCPs). Unlike the scenarios presented in the previous reports these cover four greenhouse gas concentration (not emissions) trajectories and describe possible climate futures, all of which are considered conceivable depending on how much greenhouse gases are emitted in the years to come. The four RCPs, RCP2.6, RCP4.5, RCP6 and RCP8.5, are named after a possible range of radiative forcing values in the year 2100 (+2.6, +4.5, +6.0, and +8.5 W/m², respectively).

The advantage of using concentrations in the atmosphere instead of emission pathways is that you avoid uncertainties related to the global carbon cycle.

In the scenario with the lowest greenhouse gas concentrations (RCP2.6), global mean temperatures are expected to increase by 0.3–1.7°C compared to the period (1986–2005) at the end of the century. For the RCP8.5 scenario a warming of 2.6–4.8°C is expected. In all scenarios heating is expected to be greatest in the Arctic region and larger over land than over the oceans (figure).

Figure: Maps of CMIP5 multi-model mean results for the scenarios RCP2.6 and RCP8.5 in 2081–2100 of  annual mean surface temperature change,  average percent change in annual mean precipitation.

Even under the RCP2.6 scenario the Arctic sea ice will reduce by half (-43 per cent) in summer by the end of the century and almost disappear (-94 per cent) under the RCP8.5 scenario. Spring snow cover in the northern hemisphere is also expected to shrink by 15–55 percent for RCP2.6 and by 35–85 per cent for RCP8.5.

Global sea level rise is affected directly by the melting of glaciers and ice sheets, but also by thermal expansion when oceans get warmer. Oceans have accumulated 90 per cent of the additional energy added to the climate system over the past four decades (1971–2010). Uncertainties here are rather larger, but the rate of sea-level rise will probably increase more than we have seen in the past 40 years. Under a low concentration pathway (RCP2.6) it is expected to rise by 0.26–0.40 metres in the coming century and if greenhouse gas concentrations are allowed to increase more (RCP8.5) sea levels would rise by 0.45–0.82 over the same period.

Changes in the global water cycle are even harder to predict, but we can expect that the contrast between wet and dry regions and dry and wet seasons will be greater as the climate gets warmer, though there will be regional exceptions. More instances of extreme rainfall are also to be expected as temperatures increase.

During spring the IPCC Working Group II and Working Group III will release their reports. A synthesis report of the entire fifth assessment cycle will finally be presented in October 2014.

Kajsa Lindqvist

Report: Climate Change 2013: The Physical Science Basis, IPCC Working Group I Contribution to AR5.