Paris Agreement made progress, but 2°C warming limit takes much more.

With nations gathering in Marrakech to continue talks on how to reduce greenhouse gas emissions and limit global warming, a new UN report is another stark reminder of the work to be done. This latest “Emissions Gap Report” highlights the remaining differences between the talk that produced last year’s landmark Paris Agreement and the walk required to reach those goals.

The stated goal of international talks has long been to limit global warming to 2°C above pre-Industrial Revolution temperatures. But at the behest of some countries, including low-lying island nations, an aspirational target of just 1.5°C was added in Paris. The emissions-reduction pledges made as a part of that agreement finally steered the world away from a “business-as-usual” track that could have increased the Earth's temperature by 4°C or more by the end of the century. But everyone knows that staying below 2°C will take a lot more effort. Staying below 1.5 °C seems like a pipe dream.

However, the Paris Agreement includes several milestones between now and 2030. Countries are expected to take stock of their progress toward their pledges and “ratchet up” emissions cuts. And a scientific report was commissioned to clarify what it would take to stay below 1.5°C—and how the impacts of that much warming compare to 2 or 3°C.

That report is not due until 2018. In the mean time, a new Emissions Gap Report from the United Nations Environment Programme updates us on just how far we are from hitting the 2°C or 1.5°C goals.

Taking a look at the emissions from the last couple years, the report concludes that “global greenhouse gas emissions continue to grow, and while the indications are encouraging that the growth rate of global carbon dioxide emissions from fossil fuel use and industry is slowing, it is still too early to say whether this is likely to be permanent.”

https://cdn.arstechnica.net/wp-conte..._emissions.png

Altogether, current emissions total around 53 billion tons of CO2 per year (that includes other greenhouse gases converted to a CO2-equivalent mass). In order to stay below 2°C warming, that would have to start dropping pretty much immediately and fall to about 42 billion tons per year by 2030. Pledged emissions cuts only keep us static and would still have us emitting about 53 billion tons in 2030.

If we wanted to stay below 1.5°C, emissions would obviously have to fall faster, dropping down at least to 39 billion tons per year by 2030. That trajectory would have to continue, with the world’s net greenhouse gas emissions becoming negative shortly after 2050. That is, we would have to remove more CO2 from the air (using various technologies) than we released. Even then, this scenario would probably allow temperatures to peak a little higher than 1.5 °C before dropping below the goal by the end of the century.

The 2°C scenario is a little less extreme, but also necessitates net negative greenhouse gas emissions before 2100. Some scientists have criticized these scenarios as overly optimistic given how much they rely on untested techniques to remove atmospheric CO2.

As for the progress made by existing emissions pledges, the report includes its own estimate of the temperature impact. Assuming that post-2030 actions receive the “same level of effort” (rather than less or more), we’d be on a path that would see about a 3°C warmer world. Compared to the consequences of 4°C or more, that’s excellent news.

Getting from a 3°C path down to a 2°C (or less) path would require a course correction in the very near future. Even with the pledged emissions cuts, the “budget” of greenhouse gas we can emit without going above 2°C will be nearly spent by 2030—and the 1.5°C budget will be gone.

So if countries intend to follow through on the goals they expressed in the Paris Agreement, they had better get ready to ratchet up their efforts.