The second part of the serial highlights the IPCC’s notion that, regardless of action taken now to reduce emissions, the climate will change until around the middle of this century
Regardless of future emissions, we are already committed to further warming, largely due to past emissions and inertia in the climate system. Globally, most greenhouse gas emissions due to human activities have come from just a few countries. Total emissions since 1970 have continued to rise, and emissions between 2000 and 2010 have been the highest yet.
The IPCC warns that if global society continues to emit greenhouse gases at current rates, the average global temperature could rise by 2.6–4.8°C by 2100 (according to the IPCC’s highest emissions scenario).
The figure illustrates projected warming under a low-emissions scenario, a high-emissions scenario and two mid-range scenarios, and the temperature changes associated with each. Whether global society continues to emit greenhouse gases at today’s rate, or cuts greenhouse gas emissions sharply now, does not make a big difference in terms of climate impacts in the next few decades.
Curbing emissions to maintain global temperatures below 2°C would need urgent action at global level. However, the benefits to the global climate – and societies and ecosystems that depend on it – will only emerge in the latter half of the century. The IPCC lists the many reasons why mitigation action must start now and the kinds of immediate benefits it can deliver (see page 20). In contrast, taking action on adaptation today delivers many immediate benefits. But, there are limits to adaptation. For this reason, both adaptation and mitigation are needed; they each deliver benefits but over different timeframes.
In assessing future climate change, the Fifth Assessment Report presents four scenarios, known as Representative Concentration Pathways (RCPs). The scenarios show the result of different levels of emissions of greenhouse gases, from the present day to 2100, on global warming. IPCC does not indicate which policy and behavioural choices society could make that would lead to the scenarios.
In all scenarios, carbon dioxide concentrations are higher in 2100 than they are today. The low-emissions scenario assumes substantial and sustained reductions in greenhouse gas emissions. The high-emissions scenario assumes continued high emissions. The two intermediate scenarios assume some stabilisation in emissions.
In the next few decades, warming will be the same in all scenarios. Regardless of action taken now to reduce emissions, the climate will change until around the middle of this century. In the longer term, in all except the low-emissions scenario, global warming at the end of the 21st century is likely to be at least 1.5°C.In the two higher emissions scenarios, global warming is likely to be 2°C. In the second lowest emissions scenario, global warming is more likely than not to be 2°C. Warming will continue beyond 2100 under all emissions scenarios except the lowest and will continue to vary between years and between decades.