Dangerous Climate Change – How Close Are We?

Monthly Climate Update:  August 2017

Jeff Obbard, Ph.D.*

*Jeff is a Professor of Environmental Science, currently based at Qatar University. He was previously based at the National University of Singapore, where he served as Director for the Sustainable Development & Water Alliance, and as Research Director for the Tropical Marine Science Institute. He has been identified in Singapore as a Top 100 Global Sustainability Leader, and is a recipient of the United Nations Mondialogo Award on sustainable development. Jeff was an invited Expert Reviewer for the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report.

This column is the start of a new series in ABC Carbon which provides monthly update on the latest climate data pertaining to measurements of average mean global surface temperatures (AMGST), and the prevailing atmospheric concentrations of the main greenhouse gases i.e. carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O).

Data is sourced from instrumental records (not computer models), as reported by the National Oceanic & Atmospheric Administration (NOAA), the Goddard Institute of Space Studies (GISS) of the National Aeronautical and Space Administration (NASA), the Earth System Research Laboratory (ESRL), and the Earth Institute at Columbia University, USA – and elsewhere, as appropriate.

As well, as providing information on climate data trends, this first column puts the data into the context of climate change policy with respect to the Paris Climate Agreement (PCA), which entered into force on 4 November 2016. The central goal of the PCA is to limit AMGST rise this century to below
two degrees centigrade (2^oC) above pre-industrial levels, and to pursue efforts to limit the increase to no more than 1.5^oC.  Reference is also given to the 1992 United Nations Framework Convention on Climate Change (UNFCC), where nations agreed to reduce GHG emissions so as to prevent ‘dangerous’ anthropogenic (human) interference with the Earth’s climate system.

The graph below shows recent monthly mean CO₂ concentrations, as measured at the ESRL at Mauna Loa, Hawaii. The last four complete years of the Mauna Loa CO₂ data set, plus the current year (2017) are shown.

Note: In the figure, the dashed red line with diamond symbols represents the monthly mean values, centred on the middle of each month. The black line with the square symbols represents the same, after correction for the average seasonal cycle.

The average atmospheric CO₂ concentration for August 2017 (as released by NOAA on 11 September) was 405.07 parts per million (ppm). This compares to 402.25 ppm in August 2016, an annual increase of 2.82 ppm.  The annual rate of increase of CO2 in the atmosphere 10 years ago (2007) was 2.27 ppm, and 20 years ago (1997) was 1.93 ppm indicating that the rate of accumulation of CO₂ in the atmosphere is accelerating.

August 2017 was the second warmest August on record since reliable measurements began in 1880 at+ 0.85C warmer than a 1951-1980 average baseline temperature.  August 2016 was the warmest August ever recorded   at +0.99C. Although it is still too early to rank the 2017 as a whole, 2016 AMGST was +1.24^oC  above a 1880-1920 average baseline, and was the warmest year on the instrumental record (see Figure 1).  In terms of a global warming trend, then 2015 was the second warmest year on record and 2014 the third, where 16 of the last 17 warmest years on record have all occurred in the 21^st century.

Figure 1: Average mean global surface temperature relative to 1880-1920 mean

Source:  Goddard Institute of Space Studies, NASA.

CO₂ is the main GHG associated with fossil-fuel combustion and global warming, and also the most long-lived in the atmosphere with a residence time measured in decades to centuries. The other main GHGs found in the atmosphere, apart from short-lived water vapour, include: methane (CH₄) which has a global warming potential (GWP) relative of about 32 times stronger than CO₂ (over 100 years) but a shorter atmospheric residence time of about 10 years, and; nitrous oxide (N₂O) which has a GWP of about 280 times stronger than CO₂ and an atmospheric residence time of over 100 years.

CH₄ and N₂0, although potent GHGs, occur at much lower atmospheric concentrations than CO₂ – but both are now present at elevated concentrations relative to pre-industrial times. The concentration of CH₄ in the atmosphere in June 2017 (last updated by NOAA on 05 September) was 1843.4 parts per billion (ppb), versus 1837 ppb in June 2016. This compares to less than 900 ppb of atmospheric CH₄ in pre-industrial times i.e. an increase of over 100%.  Atmospheric N₂0 concentrations are are now about 330ppb versus 273 ppb for pre-industrial  times i.e. an increase of over 20%. By combining the contribution of all GHGs to atmospheric warming and then expressing it as an equivalent CO₂ concentration, then latest available data set for atmospheric GHGs (NOAA, ESRL 2016) indicates a CO₂ equivalent (CO₂e) GHG concentration in the atmosphere of 489ppm CO₂e – i.e. in excess of the 450 ppm CO₂e associated with capping AMGST to 2^oC by 2100 (50% probability).

Based on the best linear fit of decadal AMGST of 0.17^oC per decade (1970-2017 data, see Figure 2), and a conservative assumption that the rate of increase remains constant going forward, then the 1,5⁰C limit of the PCA can be expected to be breached in about 15 years’ time i.e. 2032 and the 2oC limit in about 45 years’ time i.e. 2062.  Current national pledges to reduce carbon emissions under the PCA indicate that the National Determined Contributions (NDCs), as submitted by signatory nations to the agreement will, according to the International Energy Agency, result in an AMGST increase of 2.7^oC by the year 2100, and 3^oC thereafter.

These estimates do not include the denigration of the integrity of the PCA, as prompted by the decision, in June 2017, of the USA, the world’s second largest emitter of CO₂ to atmosphere, to withdraw from the agreement.  Further, climate sensitivity data i.e. the measure of the response of the global climate system to a given radiative forcing by atmospheric GHGs, coupled with paleoclimate (earth history) evidence, suggest that current levels of AMGST warming i.e. +1.28^oC are sufficient to trigger long-term positive feedbacks in the climate system and accelerate global warming. Such feedbacks include the disintegration of the world’s ice-sheets, and the melting of carbon-rich Arctic permafrost. Indeed, paleoclimate data from the Eemian interglacial period, from about 126,000 years ago indicates that equilibrium AMGST for similar atmospheric GHG concentrations in the atmosphere that we see today resulted in sea levels between 6 and 9 metres higher than present.

Although the physics of climate change is not in question scientifically, the time taken to reach radiative equilibrium for an energy imbalance in the earth-atmospheric system due to increased radiative GHG forcing is less certain. In its 2014 fifth assessment report (AR5), the Intergovernmental Panel on Climate Change (IPCC) reported sea-level rise estimates of up to 0.5m by the end of the century. However, paleoclimate evidence suggests that multi-meter sea level change is possible over a period of a century or more, due to positive feedback mechanisms. This has led to less-conservative predictions of up to several metres of sea level rise this century or shortly thereafter due to ocean thermal expansion, glacier melt and the collapse of land based ice-sheets, namely Greenland and the Western Antarctica.  If indeed positive feedback mechanisms are triggered by current and future increases in AMGST, then this has major implications for low-lying island state nations, such as Singapore.

With each passing month, it is becoming ever more clear that the world needs an ambitious and globally coordinated action to reduce GHG emissions if it is to avoid the ‘dangerous’ climate change referred to by the UNFCC twenty-five years ago.