New hope for global warming?

21 September 2017

Scientists have a new best estimate for how much carbon dioxide we can emit and still limit global warming to the 1.5°C pledged in Paris in 2015. The new estimate gives us a larger carbon budget, but whether that translates to new hope depends on how the message is recieved.

Over the last 10,000 years or so average global temperatures have fluctuated by half a degree, give or take. But for the first time in human history we are up by almost 1°C relative to pre-industrial levels.

The main driver of this warming is accumulation of carbon dioxide (CO2) in the atmosphere, which traps the Sun's energy. This is also known colloquially as ‘the greenhouse effect’. Limiting CO2 emissions, by imposing a ‘carbon budget’, is regarded as a key means to limit global warming. Traditionally, the stated goal of climate talks has been to limit warming to 2°C above pre-industrial levels, and scientists have estimated what the carbon budget for that limit might be. However, this 2°C limit may be more of a political threshold than a scientific one.

Scientifically speaking, we are already feeling the effects of a 1°C warming. As global average temperatures have shifted, so too has the extreme ends of the thermal spectrum. Climate change is leading to loss of coral reefs, more intense storms and hurricanes, melting permafrost, and rising sea levels. Further increases in global warming will exacerbate these problems further.

The goal of the 2015 Paris Climate Agreement was 'holding the increase in global average temperature to well below 2°C  above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C’. This ambitious target surprised scientists because estimates at the time of the remaining carbon budget required to limit warming to 1.5°C equated to approximately 4 year's current emissions and was considered to be almost a geophysical impossibility.

Now, publishing in Nature Geoscience, scientists have a new best estimate for what the remaining carbon budget might be to limit warming to 1.5°C. The authors calculated this carbon budget relative to today, as opposed to calculating the carbon budget relative to pre-industrial levels, as has been done historically. The idea behind this new approach is that the shorter time scale of a ‘relative to today calculation’ limits the amount of error that creeps into the model compared to the longer time scale of a ‘relative to pre-industrial’ calculation.

According to study author Joeri Rogelj, “what we find if we take into account where we are today, we get to a budget of around 700 to 800 billion tonnes of carbon dioxide; this corresponds to about 20 years of current emissions.”

The authors do not claim 100% certainty; the estimated carbon budget comes with a 66% certainty and working this out is no mean feat. These calculations need to be done on some of the most powerful computers we have today and once all the parameters are meticulously set, these models run for months to come up with an answer.

Understanding these parameters also helps place the current result in context. For example, these parameters include non-CO2 contributors to global warming like emissions of nitrous oxide and methane. This latest model assumes reductions in these non-CO2 contributors, a scenario known in the field as an ‘ambitious mitigation strategy’.

There are a few ways to think about the rationale for assuming ambitious mitigation of non-CO2 contributors to global warming.

On one hand there are reasons to think that these non-CO2 contributors would not decline but actually increase. With an increasing global population, the increased demand for food will need to be met by the agricultural industry, which is currently the main source of these non-CO2 contributors and responsible for an estimated one third of total human-related or 'anthropogenic' greenhouse gas emissions.

On the other hand, we do expect rapid technological development in the coming decades and this development can be steered towards reducing greenhouse gas emissions. There are promising signs of this, for example a group of Scottish researchers recently won the 2017 PLoS Genetics Research Prize for work that could pave the way for low-emission cattle.

Ultimately, the authors have elected to err on the side of optimism. This comes with potential pros and cons. One benefit of this approach is that, psychologically it steps us back from a tipping point. If people believe that we have gone past a point of no return then they may also believe that any efforts towards conservation would be meaningless.

There is a risk though that people may view this as a reason to no longer pursue aggressive reduction of carbon emissions. But this would be a misinterpretation. As Joeri Rogelj says, “our research really put the possibility of limiting warming to 1.5°C back on the table and I think that’s a hopeful message, but that doesn’t really mean that the pressure is off; it still requires actions way beyond the pledges that are currently on the table by countries and global carbon dioxide emissions need to be reduced immediately, to become zero by mid-centaury and that’s an incredibly challenging feat.”

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