US National Climate Assessment Leaked

From The New York Times 7 August 2017 – “A final draft report by scientists from 13 federal agencies concludes that Americans are feeling the effects of climate change right now. The report was completed this year and is part of the National Climate Assessment, which is congressionally mandated every four years.” It was uploaded to a nonprofit internet digital library in January but received little attention until it was published by The New York Times. Sources have said the leak was due to fears that it would otherwise be suppressed by the Trump administration.

The report I have looked at is identified as Final Clearance 28 June 2017. This is no small report, a 60.6MB file running to 669 pages, with input from 3 coordinating lead authors, 29 lead authors and 18 contributing authors. The following points are derived primarily from the executive summary which has been designed to be accessible to a wider audience than the remainder of the report targeted at the scientific community.

It is regarded as extremely likely (defined as 95 to 100% certainty) that human influence has been the dominant cause of the observed warming since the mid-20th century. The following chart shows the temperature anomalies since 1880. For the period 1951 to 2010 the likely (66 to 100%) human impact is assessed as 0.6 to 0.8°C.  Since the last NCA was published 2014 has been reported as the warmest year to date but then exceeded in turn by 2015 and 2016. The human effect from 1750 is estimated at about 2.2 watts/m2 whereas that from solar irradiance is put at less than 0.1 watts/m2 and volcanic at close to -0.1 watts/m2.  As the report points out the effect of volcanic emissions is always negative, reaching as high as -11.6 watts/m2 immediately after the eruption of Mt Tambora in 1815. It does not though, from what I have seen in the executive summary, make the point that a reduction in volcanic activity would result in some element of global warming.

The following map indicates the areas most affected in comparing surface temperatures 1986 to 2016 compared to 1901 to 1960. [I can only assume the the effect would be much more marked if the latest figures had been limited to the latest decade.] Note that the temperature change is in °F.

The report is understandably very much geared towards North America but some impacts such as sea level changes are a global phenomenon. Very high confidence is attributed to the statement that the world’s oceans have absorbed about 93% of the excess heat caused by greenhouse gas warming since the mid-20th century, increasing water temperatures at all depths. For surface waters this is calculated to be around 0.7°C. The rise in mean sea levels since 1900 is reported as some 18cm with about 40% of this occurring in the last 20 years. This increase indicates a rise from historically (last 2500 years) low sea levels to the highest. Almost completely ice free Arctic seas are predicted as very likely by 2050.

The increased CO2 in the atmosphere to above 400 ppm is stated with high confidence as bringing it in line with that pertaining 3 million years ago when global temperatures and sea level were higher than today.

While the report considers known factors with some certainty they also acknowledge the existence of other factors:

  • The impact of any geoengineering.
  • The possibility that land areas will move from being a net carbon sink to a carbon source in view of the uncertainty of the trajectory of land cover.
  • Elements of the earth’s systems that currently cannot be modelled well, for example changes in land use.
  • Compound events occurring simultaneously or sequentially which could cross some threshold in the climate system and lead to major impacts.
  • Self-reinforcing cycles which could lead to major changes, eg fundamental changes in atmospheric or ocean circulations (extending to what my US readers will readily recognise as unknown unknowns).

If greenhouse gas concentrations were to be stabilised at current levels it predicts continued warming with high confidence, and that this will be at least a further 0.6°C, with medium confidence. As to the future a number of scenarios are discussed with related future rises varying from zero to around 4°C.

The report is a mine of information and I have only touched on the topics included in the Executive Summary. While there may be different views on the cause of global warming it is difficult to dismiss the mass of data confirming the extent of warming to date. Easier to dismiss are predictions of future warming and its link to CO2 emissions but it would be a brave person who would advocate taking no action. One hopes that the project is not unduly influenced by those with vested business or political interests.

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