Last month the leaders of the United States and China agreed on a deal to cut carbon emissions, after nine months of secret correspondence.

The deal commits China to capping its carbon emissions at whatever rate it hits in 2030, while the US has pledged to cut its emissions by 26 — 28 % of its 2005 levels by 2025.

As the top two carbon polluters on the planet, the US and China account for 40% of greenhouse gas emissions, meaning their actions could shape the future of Earth’s climate.

The big question is if these specific cuts they promised will actually change the fate of the planet.

Doing the maths

There’s a terrifying piece of maths out there that we can use to check the importance of this carbon agreement.

The United Nations Environment Programme (UNEP)’s 2014 Emissions Gap Report, released last month, tells us that humans can only afford to pump about 1,000 more gigatons of carbon dioxide (one gigaton equals a billion tons) into the atmosphere if we want to keep the world’s temperature rise under 2 degrees.


Beyond this point, many scientists agree Earth could suffer a domino-like series of catastrophic environmental consequences, including rapid sea level rise, food and water shortages, disease epidemics, and mass extinctions.

We crunched some numbers to see just how much of an impact the climate deal will have. Along the way, we had to solidify certain assumptions regarding how the deal’s signatories could go about fulfilling their promise.

Here’s what we found:

How much will China’s carbon increase?

It’s impossible to know the future, but in the previous decade, its emissions increased each year by around 10 per cent. The growth rate has dropped closer to 6 per cent in recent years, according to the European Commission’s 2013 report on CO2 emissions, and in 2012 — the last year for which concrete data exists — the growth rate was more like 3 per cent.

So what for the future?

It’s possible that China may try to over-inflate its carbon footprint before 203o to give themselves a big platform to start climbing down from after 2030. Or it could continue to increase at a more or less constant rate, as it has done in recent years, until 2030. The country could even take carbon-decreasing steps now, and slow its growth rate steadily, though we think this is a long shot.

We went with the extremely moderate assumption that China’s emissions will continue to increase by about 3 per cent, as they did in 2012, each year until 2030. According to the US Energy Information Administration (EIA), China’s CO2 emissions were about 8.55 gigatons in 2012. Sticking to a 3 per cent increase each year, this would put China at a whopping 14.55 gigatons a year in 2030.

How much will the US reduce its emissions by?

We also can’t be sure how — or even if — the US will go about cutting its emissions over the next 10 years, but a linear decrease between now and 2025 seems the most likely course of action, allowing the US to gradually transition down to the promised emissions levels.

President Obama has promised to cut carbon emissions down by between 26 and 28 % of their 2005 levels of nearly 6 gigatons. The EIA reports that the US’s CO2 emissions in 2012 were already a bit lower, at about 5.27 gigatons, though US carbon emissions have held pretty steady over the last five years.

We can assume emissions in 2014 will be around the same amount. From there, cutting emissions by 26 per cent of 2005 levels would put the country’s CO2 emissions at about 4.44 gigatons in 2025.

In order to decrease linearly from 5.27 gigatons to 4.44 gigatons by 2025, the US will have to cut approximately 0.075 gigatons of CO2 each year.

This plan will spare the atmosphere nearly 5 gigatons of CO2 over the next 10 years.

Will we overrun our CO2 budget under the new deal?

Unfortunately, the US and China aren’t going to save the world by themselves. Under the above conditions, and assuming both countries meet their goals and are able to hold their emissions constant thereafter, they will still jointly pour more than 600 gigatons of carbon dioxide into the atmosphere between now and 2050. This course would blow nearly two thirds of the 1,000-gigaton budget by mid-century, leaving less than 400 gigatons for the rest of the world’s nations to spend amongst themselves.

Since the rest of the world, excluding China and the US, accounts for about 60 per cent of global emissions, this would put the planet on track to blow past the 1,000 gigaton budget in the next few decades.

The best-case scenario would be for China and the US — along with the rest of the world — to continue steadily cutting their emissions through the rest of the century. In its latest report, the Intergovernmental Panel on Climate Change (IPCC), warned that the only way to avoid to stay within the 2 degree limit would be for the world to become carbon-neutral by 2100.

If countries — particularly major carbon polluters like the US, China, India, Russia, and the European Union — take the problem seriously and keep cutting their emissions throughout the coming decades, the world still has a shot at keeping temperatures in check. Some have already risen to the challenge. In addition to the US-China deal, the European Union recently pledged to cut its emissions by 40 per cent between now and 2030.

But even with these promises, experts are increasingly pessimistic that the 2-degree goal is still attainable.

As world leaders convene this week and next week at the UN Framework Convention on Climate Change in Lima, Peru, to draft a plan for cutting global emissions, pressure will be high to come up with a strategy that is both feasible and capable of saving the planet from the worst climate change effects.