A few weeks ago cliscep had a piece Trump, climate and the future of the world that looked at the immediate reactions to the surprise victory in the US Presidential election amongst the climate community. Brad Keyes noted Jo Romm’s piece will President Trump pull the plug on a livable climate?. To support this Romm stated

Indeed, one independent firm, Lux Research, projected last week that “estimated emissions would be 16 percent higher after two terms of Trump’s policies than they would be after two terms of Clinton’s, amounting to 3.4 billion tons greater emissions over the next eight years.”

There is a little graph to sort of back this up.

Whilst Romm then states two reasons why he does not think emissions will rise so much (Trump will cause a massive recession and will not win a second term) he then states the Twitter quote:-

That said, the damage and delay that even a one-term President Trump could do will make the already difficult task of keeping total warming well below 2°C essentially impossible.

So a difference of much less than 3.4 GtCO2e over eight years will make keeping total warming well below 2°C essentially impossible.
Before looking at the evidence that contradicts this, there are even more bizarre claims made by the expert climate scientists at RealClimate. They use a different graph which is probably a couple of years old and explain:-

Here are some numbers. Carbon emissions from the United States have been dropping since the year 2000, more than on-track to meet a target for the year 2020. Perhaps with continued effort and improving technology, emissions might have dropped to below the 2020 target by 2020, let’s say to 5 gigatons of CO2 per year (5000 megatons in the plot). In actuality, now, let’s say that removing restrictions on energy inefficiency and air pollution could potentially lead to US emissions by 2020 of about 7 gigatons of CO2. This assumes that future growth in emissions followed the faster growth rates from the 1990’s.
Maybe neither of these things will happen exactly, but these scenarios give us a high-end estimate for the difference between the two, which comes to about 4 gigatons of CO2 over four years. There will also probably be extra emissions beyond 2020 due to the lost opportunity to decarbonize and streamline the energy system between now and then. Call it 4-6 gigatons of Trump CO2.
This large quantity of gas can be put into the context of what it will take to avoid the peak warming threshold agreed to in Paris. In order to avoid exceeding a very disruptive warming of 1.5 °C with 66% probability, humanity can release approximately 220 gigatons of CO2 after January, 2017 (IPCC Climate Change 2014 Synthesis report, Table 2.2, corrected for emissions since 2011). The 4-6 Gtons of Trump CO2 will not by itself put the world over this threshold. But global CO2 emission rates are now about 36 gigatons of CO2 per year, giving a time horizon of only about six years of business-as-usual (!) before we cross the line, leaving basically no time for screwing around. To reach the catastrophic 2 °C, about 1000 gigatons of CO2 remain (about 20 years of business as usual). Note that these estimates were done before global temperatures spiked since 2014 — we are currently at 1.2 °C! So these temperature boundaries may be closer than was recently thought.

RealClimate come up with nearly twice the difference made by Joe Romm / Lux Research, but at least admit in the final paragraph that whoever won would not make much difference.
There are two parts to putting these analyses into context – the US context and the global one.
In the USA emissions have indeed been falling since 2000, this despite the population growing. The rate of decline has significantly increased in the years of the Obama Presidency, but for reasons quite separate from actions to reduce emissions. First there was the credit crunch, followed by the slowest recovery in US history. Second, the high oil price encouraged emissions reductions, along with the loss of energy-intensive industries to countries with lower energy costs. Third is that the shale gas revolution has meant switching from coal to gas in electricity production.
But the global context is even more important. RealClimate does acknowledge the global figure, but only mentions CO2 emissions. The 36GtCO2 is only two-thirds of total greenhouse gas emissions of about 55GTCO2e and that figure is rising by 1-2% a year. The graph – reproduced from the USA INDC submission to the UNFCCC – clearly states that it is in million tonnes of carbon dioxide equivalent. What is more, these are vague policy proposals, that President Obama would have been unable to get through Congress. Further, most of the proposed emission reductions were through extrapolating trends that of what has been happening without any policy intervention.
If the 1.5°C limit breached from 220 GtCO2e of additional emissions, it will be breached in the run-up to Christmas 2020. The 1000 GtCO2e for the 2°C limit was from 2011. By simple arithmetic it is now below 800GtCO2e with about 15 years remaining if (a) a doubling of CO2 levels (or equivalent GHG gases) leads to 3°C of warming (b) the estimated quantity of emissions to a unit rise in atmospheric gas levels is correct and (b) the GHG gas emitted is retained for a very long period in the atmosphere.
Even simple arithmetic is not required. Prior to the Paris talks the UNFCCC combined all the INDCs – including that of the USA to cut emissions as shown in the graph above – were globally aggregated and compared to the approximate emissions pathways for 1.5°C and least-cost 2°C warming. The updated version, post-Paris is below.

The difference Donald Trump will make is somewhere in the thickness of the thick yellow line. There is no prospect of the aimed-for blue emissions pathways. No amount of ranting or protests at the President-elect Trump will change the insignificant difference the United States will make with any politically-acceptable and workable set of policies, nor can make in a country with less than a twentieth of the global population and less that one seventh of global emissions.

Kevin Marshall