Warming can be limited to 1.5°C

Illustration: © Lars-Erik Håkansson

There are reduction pathways that would make a global target of below 1.5°C possible, but they require negative CO₂ emissions in the second half of this century.

On behalf of Climate Action Network Europe and AirClim, Climate Analytics has analysed the adequacy and feasibility of limiting global warming to below 1.5°C. The report describes the effects of climate change for people and the environment at 0.8, 1.5, 2 and 4 degrees C global warming, and states that in the past century and in particular the last few decades the world has seen signals of anthropogenic climate change emerging as diverse as rapid sea-ice thinning in the Arctic, monthly and seasonal temperature extremes, extreme droughts in the Mediterranean, decline of coral reefs and negative effects on agricultural yields.

The report says that hypothetically, if all emissions were to be eliminated immediately, delays in the climate system and abrupt changes in atmospheric radiative forcing would let warming continue to rise to a best-guess level of 1.2°C above the preindustrial level, before embarking on a gradual decline.

It is of course not possible to stop global emissions of greenhouse gases abruptly, but Climate Analytics identifies emission reduction pathways which would at least make a global target of below 1.5°C possible. These pathways would allow for a temporary overshooting of a greenhouse gas concentration of below 400 ppm CO₂eq, which is equivalent to around 1.5°C of warming, but in the long term concentrations would drop as a result of the proposed measures.

New energy and socioeconomic models from the scientific literature achieve the required low emission levels through the following key measures:

• Early and globally concerted mitigation, emission reductions implementefrom 2013 onwards and global emission peak by 2020 without use of nuclear energy.
• Rapid up-scaling and feasibility of large-scale bio-energy, and availability of forest sinks.
• High rates of energy efficiency improvements.
• Availability of carbon capture and storage technologies for bioenergy (BCCS).

According to Climate Analytics, long-term 1.5 and 2°C emission scenarios overlap until the 2030s, but a 1.5°C scenario requires deeper reductions in the rest of the 21st century with the following pathway:
“Constrained by actual emissions until 2010 and the limited energy-economic reduction potential until the 2020s, 1.5°C scenarios necessarily require net negative CO₂ emissions in the second half of the 21st century.

The later the emissions peak, the more CO₂ needs to be removed from the atmosphere starting around the 2050s. Due to slowly responding carbon pools in the Earth system, a large part of emitted CO₂ stays in the atmosphere for centuries, which is why emissions need to be reduced to near zero to stabilise concentrations. However, this also means that concentrations decrease only slowly, unless CO₂ is taken out of the atmosphere by human interventions. As biomass takes up carbon from the atmosphere through photosynthesis, capturing the CO₂ from biomass energy systems and storing it underground, it in effect produces useful forms of energy for society (electricity) while taking CO₂ out of the atmosphere – a negative emission. CO₂ removal also helps to limit acidification of the oceans.”

Climate Analytics explains in the report that non-CO₂ measures must never be interpreted as a means for “buying time” to allow delayed reductions in CO₂. “The probability of exceeding a 2°C warming in the 21st century more than doubles from 20% to 50%, if CO₂ reductions were delayed by just 10 years, with compensation in the near term by non-CO₂ measures. Given the slow removal of CO₂ from the atmosphere, this effect is set to linger for centuries. Also, after a delay in CO₂ reductions, energy-related CO₂ reduction rates need to be almost double those in a “least-cost” low-emission pathway with early CO₂ measures. Without these higher reduction rates to “catch up”, the CO₂ concentration and warming by 2100 will be even higher. From a multi-decadal perspective, delay scenarios have been shown to be riskier, with required faster CO₂ reductions after a 10-year delay too expensive and/or technically infeasible. The IEA’s “World Energy Outlook 2011” states that: “Delaying action is a false economy: for every $1 of investment avoided in the power sector before 2020 an additional $4.3 would need to be spent after 2020 to compensate for the increased emissions.”

Today more than 100 countries support the 1.5°C target and in the UN Climate Convention this year a review process has been initiated to possibly adopt this target and reach a decision in 2015 about a reduction pathway for greenhouse gases that would make it possible to reach this long-term global limit.

Reinhold Pape

APC 29: Adequacy and feasibility of the 1.5°C long-term global limit by Michiel Schaeffer, Bill Hare, Marcia Rocha & Joeri Rogelj fromClimate Analytics.

Link to Climate Analytics and Climate Action Tracker