Summary
Scientific knowledge about climate change is being improved all the time through research into the climate system, climate forcing, climate variations and changes, and climate effects.
The knowledge base is well established when it comes to the fundamental physics behind the greenhouse effect and the fact that the average surface temperature has increased over the past 50 years. It is also highly likely that most of the observed warming is due to human impact on the climate.
At the same time there is considerable uncertainty regarding the consequences of climate change and how much emissions need to decrease to achieve a set climate target. Knowledge about climate sensitivity is the most important factor in calculating how much greenhouse gases we can emit for a given temperature target.
Research shows that considerable emission reductions are required to achieve the two-degree target. To achieve a lower temperature target of, for instance, 1.5°C, even greater emission reductions are needed.
- To achieve the two-degree target with a probability of around 70%, it is estimated that global greenhouse gas emissions need to decrease by 50-60% from 2000 to 2050, and by almost 100% by 2100.
- To achieve a 1.5-degree target with a probability of around 70%, zero emissions are needed globally as early as 2050.
- To achieve a 1.5-degree target with a probability of around 50%, it is estimated that global greenhouse gas emissions need to decrease by around 80% from 2000 to 2050, and by almost 100% by 2100.
It is primarily the cumulative emissions of carbon dioxide and other long-lived greenhouse gases that count when it comes to the magnitude of climate change beyond 2100. The later the global emissions culminate, and the higher their level at the culmination, the greater the challenge in bringing about a sufficiently rapid ensuing rate of emission reduction. Reduced emissions of so-called short-lived climate forcers substances are important primarily in a more near-term perspective.
There are different models for how global emission reduction needs could be distributed between different regions and countries. A decision on such a distribution is not a natural sciences’ question. Rather it depends on political and other standpoints. For some countries the results vary greatly depending on the choice of distribution model. For most industrialised countries, however, the conclusion is generally the same: their emissions must decrease dramatically compared to current levels.
- Achieving the two-degree target with 70% probability requires, with a global contraction (reduced overall emissions) and convergence to an equal per capita emission level for all countries as of 2050, that emissions in Sweden decrease by approximately 70% from 2005 to 2050. The corresponding figure for the EU is approximately 80%.
- Achieving a 1.5-degree target with 70% probability requires, with a global contraction (reduced overall emissions) and convergence to an equal per capita emission level for all countries as of 2050, that emissions from 2005 to 2050 decrease by around 100% in Sweden and the EU, as well as in other countries.
- Achieving a 1.5-degree target with 50% probability requires, with a global contraction (reduced overall emissions) and convergence to an equal per capita emission level for all countries as of 2050, that emissions in Sweden and the EU decrease by over 90% from 2005 to 2050.
Net emissions of carbon dioxide from deforestation and international aviation and shipping are not included in these estimates.
Generally speaking, the more ambitious the temperature target chosen, the less the risk of serious climate effects, but the risks do not vanish with the two-degree target, nor even with a 1.5-degree target.
Since the IPCC Fourth Assessment Report (AR4) was published in 2007, new research results into climate effects have been published. In this report we have focused on sea-level rise, ocean acidification, biological diversity and the Arctic region. Compared to the AR4, the later results show that the future rise in sea level could be greater, the effects of ocean acidification on marine ecosystems more extensive, and even though various species may be able to adapt, the world’s ecosystems may be influenced by differences in different species’ vulnerability to climate change. In the Arctic, rapid changes are occurring.
All in all, the risks of serious climate effects appear to be greater than set out in AR4.
This report is based on natural sciences’ climate research since 2007. The report does not recommend a specific temperature target, a specific emission pathway or other specific policy decisions. Rather, these are subjects for political deliberations and decisions.
Conclusions
This review highlights the knowledge base for climate work from a scientific perspective, based on the assignment the Swedish government gave to SMHI at the end of May 2011 (M2011/2166/Kl). The focus is on: (i) how new knowledge and new research results influence previous conclusions about climate change and climate effects, (ii) scientific prerequisites for the two-degree target, and (iii) scientific prerequisites for a 1.5-degree target.
The main conclusions are as follows:
- The knowledge relating to climate sensitivity reported in AR4 (IPCC 2007a) still is timely. New research into how natural carbon sinks and carbon sources are influenced by climate change indicates, however, that the future net carbon uptake terrestrial systems could be less than formerly estimated.
- There are now far more studies and information about emission pathways compared to when AR4 was published. The studies inform primarily about the two-degree target, rather than even lower temperature targets.
- Measures to reduce emissions of short-lived climate forcers such as tropospheric ozone and soot may help in limiting global warming in the near term, but such measures are not sufficient to curb the warming.
- The later global emissions culminate, the lower the probability of the two-degree target being met. In order to meet the two-degree target with a relatively high probability (around 70%), global greenhouse gas emissions must peak over the next 5-10 years, and by 2050 they must have decreased by approximately 50-60% compared to 2000.
- There are different models for how global emission reductions can be distributed between different regions and countries. Such models depend on political and other standpoints.
- Projections based on a convergence of the per capita emissions in different countries to the same level by 2050 and for the two-degree target to be achieved with a probability of around 70% indicate that Swedish emissions need to decrease by approximately 20% by 2020 and by 70% by 2050 compared to 2005. The corresponding figures for the EU are approximately 25% and 80% respectively. Net emissions of carbon dioxide from deforestation and international aviation and shipping are not included in these figures.
- A lower temperature target, such as 1.5 degrees, requires far more comprehensive emission reductions and could be unachievable without a temporary overshooting of the required long-term stabilisation of greenhouse gas levels in the atmosphere.
- For a 1.5-degree target to be achieved without such an overshoot and with a probability of approximately 50%, global emissions need to start decreasing within the next few years. By the year 2050, global emissions must have decreased by 80% compared to 2000. A probability of approximately 70% involves that global emissions are around zero by 2050.
- Projections based on a convergence of the per capita emissions in different countries to the same level by 2050 and for a 1.5-degree target to be achieved with a probability of around 50%, indicate that Swedish emissions need to decrease by approximately 25% by 2020 and by over 90% by 2050 compared to 2005. The corresponding figures for the EU are approximately 30% and just over 90% respectively. Net emissions of carbon dioxide from deforestation and international aviation and shipping are not included in these figures.
- There is uncertainty regarding the climate effects under different temperature targets, but it is well-established that climate effects in certain regions, such as the Arctic, could be extensive even if the two-degree target is reached. Reducing global warming reduces the risk of climate effects, but even if the two-degree target is met the sea level will rise, ocean acidification will increase and important impacts on biological diversity can be expected.