The goal of international climate negotiations is “to avoid dangerous atmospheric concentrations of greenhouse gases.” In 2010, Parties to the United Nations Framework Convention on Climate Change formally recognized the “long term goal” of the convention was to hold the increase in global average warming to below 2 degrees Celsius above pre-industrial levels.

Is 2 degrees Celsius therefore the safe limit above which climate change becomes “dangerous”? A UN expert dialogue of more than 70 scientists, experts, and climate negotiators recently released a final report concluding that 2 degrees Celsius is “inadequate” as a safe limit.

The report will feed into a review of the 2 degrees Celsius limit, including discussions on a tougher 1.5 degrees Celsius warming limit in the new climate agreement expected in Paris in December.

So, what does the evidence say?

What’s the difference between 1.5 and 2 degrees Celsius?

It is well known that the risks of climate change can be significantly reduced if warming is limited to well below 2 degrees Celsius.

However, the scientific literature related to 1.5 degrees Celsius is scarce, as the Intergovernmental Panel on Climate Change (IPCC) compares differences along 2 degrees Celsius and 4 degrees Celsius pathways—somewhat at odds with the current policy debates over temperature limits and danger thresholds.

Global average warming is just that—an average. Regional warming and vulnerability to climate impacts will vary significantly. Therefore the difference in projected risks between 1.5 degrees Celsius and 2 degrees Celsius of warming is particularly important for highly temperature-sensitive systems, such as the polar regions, high mountains and the tropics, and low-lying coastal regions.

At 2 degrees Celsius the very existence of some atoll nations is threatened by rising sea-levels. Limiting warming to 1.5 degrees Celsius may restrict sea level rise below one meter.

Yet even at 1.5 degrees Celsius warming, regional food security risks are significant. Africa is particularly vulnerable, with significant reduction in staple crop yields in some countries. Current levels of warming are already causing impacts that many people will not be able to adapt to—more scope for adaptation would exist at 1.5 degrees Celsius, especially in the agricultural sector.

Can we limit warming to 1.5 degrees Celsius?

The 2 degrees Celsius warming limit or “guardrail” has long been controversial. It was rejected by many developing countries at Copenhagen and over two thirds of Parties to the Convention call for a 1.5 degrees Celsius limit. So is this ambitious temperature limit still within reach?

The carbon budget approach—adopted by the IPCC in its latest report—defines the amounts of cumulative CO2 emissions which will drive warming to a given global temperature limit. The most stringent IPCC scenario gives a remaining (from 2011) carbon budget of 1,000 billion tonnes of CO2, for a “likely” chance of keeping global temperature within 2 degrees Celsius.

Yet whether a lower temperature limit is still within reach, and the pathway to get there, is debated. The more ambitious mitigation scenarios reported by the IPCC are characterized by overshooting the budget and then removing greenhouse gases from the atmosphere. This usually means relying on bioenergy plus carbon capture and storage (burning biomass for energy, removing the CO2, and then storing it underground) to remove carbon from the atmosphere—which comes with its own risks.

1.5 degrees Celsius pathways which do not rely on negative emissions depend on a much lower remaining budget. Even a 50 percent chance of keeping below 1.5 degrees Celsius requires immediate and radical emission reductions. This would mean unprecedented annual rates of decline which are not in line with current levels of energy consumption or ideas of economic growth.

Others suggest that, for fossil fuel emissions and for developed economies, there is already no carbon budget left at all.

Moreover, this discussion doesn’t account for aerosol and particulate pollution masking the impact of greenhouse emissions, which could mean an additional 0.8 degrees Celsius of warming is already “locked in,” increasing the scale of the challenge.

The UNFCC expert group recognized that limiting global warming to below even 2 degrees Celsius necessitates a radical transition, not merely a fine-tuning of current trends, yet such radical emissions reduction pathways are so far excluded from IPCC assessment, leaving policy makers with little evidence on the impacts and feasibility of lower targets.

Where to from here?

The group concluded that the world is not on track to achieve the long-term global goal of 2 degrees Celsius, noting that the longer we wait to bend the curve of global greenhouse gas emissions, the steeper we will have to bend it down later.

The report will feed into discussions in relation to a decision on the global goal, expected at the Paris congress, with the report noting that limiting global warming to below 1.5 degrees Celsius would come with several advantages in terms of coming closer to a safer “guardrail.”

However, the expert group falls short of recommending a 1.5 degrees Celsius goal, arguing that the science on a 1.5 degrees Celsius warming limit is less robust, despite presenting evidence that, in some regions, very high risks are projected for warming above 1.5 degrees Celsius. 

The idea that the 2 degrees Celsius threshold is not safe is not new. Ten years ago prominent climate scientist James Hansen said the 2 degrees Celsius threshold “cannot be considered a responsible target” and subsequently called for a 1 degrees Celsius limit, with a carbon budget of just 500 Gt.

Only a few weeks ago, Hansen told ABC breakfast radio that it was crazy to think of 2 degrees Celsius as a safe limit.

Others have joined the fray, challenging the acceptance of high probabilities of exceeding 2 degrees Celsius, and risky mitigation pathways to get there. Kevin Anderson of the Tyndall Centre in Britain has said that 2 degrees Celsius represents a threshold, not between acceptable and dangerous, but between “dangerous” and “extremely dangerous” climate change.

According to the IPCC’s budget numbers, only the very ambitious 1.5 degrees Celsius pathway also gives us a high probability of holding warming even below 2 degrees Celsius. After decades of procrastination, limiting warming to 1.5 degrees Celsius, or even increasing the probabilities of not exceeding 2 degrees Celsius, will now require action “faster than most policy makers conceive is possible.”

The Conversation

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