A matter of degrees: why 2C warming is officially unsafe

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 recognised the "long term goal" of the convention was to hold the increase in global average warming to below 2C above pre-industrial levels.

Is 2C 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 2C is "inadequate" as a safe limit.

The report will feed into a review of the 2C limit, including discussions on a tougher 1.5C 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 2C?

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

However, the scientific literature related to 1.5C is scarce, as the Intergovernmental Panel on Climate Change (IPCC) compares differences along 2C and 4C 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 1.5C and 2C 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 2C the very existence of some atoll nations is threatened by rising sea-levels. Limiting warming to 1.5C may restrict sea level rise below 1 metre.

Yet even at 1.5C 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 1.5C, especially in the agricultural sector.

Can we limit warming to 1.5C?

The 2C 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 1.5C limit. So is this ambitious temperature limit still within reach?

The carbon budget approach - adopted by the Intergovernmental Panel on Climate Change (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°C.

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 characterised 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.

1.5C pathways which do not rely on negative emissions depend on a much lower remaining budget. Even a 50% chance of keeping below 1.5C 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 0.8C of warming is already "locked in", increasing the scale of the challenge.

The UNFCC expert group recognised that limiting global warming to below even 2C 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 2C, 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 1.5C would come with several advantages in terms of coming closer to a safer "guardrail".

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

The idea that the 2C threshold is not safe is not new. Ten years ago prominent climate scientist James Hansen said the 2C threshold "cannot be considered a responsible target" and subsequently called for a 1C 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 2C safe limit.

Others have joined the fray, challenging the acceptance of high probabilities of exceeding 2C, and risky mitigation pathways to get there. Kevin Anderson of the Tyndall Centre in Britain has said that 2C 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 1.5C pathway also gives us a high probability of holding warming even below 2C. After decades of procrastination, limiting warming to 1.5C, or even increasing the probabilities of not exceeding 2C, will now require action "faster than most policy makers conceive is possible".

Kate Dooley, PhD candidate, Australian German Climate & Energy College, University of Melbourne and Peter Christoff, Associate Professor, School of Geography, University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.