The Paris Agreement
The Paris Agreement central aim is to strengthen the global response to the threat of climate change by keeping a global temperature rise this century well below 2 degrees Celsius above pre-industrial levels and to pursue efforts to limit the temperature increase even further to 1.5 degrees Celsius.UnitedNations, Climate Change: The Paris Agreement
A false assumption
The IPCC’s report, Global warming of 1.5°C, Summary for Policymakers, says:
B.2 By 2100, global mean sea level rise is projected to be around 0.1 metre lower with global warming of 1.5°C compared to 2°C (medium confidence). Sea level will continue to rise well beyond 2100 (high confidence), and the magnitude and rate of this rise depend on future emission pathways.
B.2.3 Increasing warming amplifies the exposure of small islands, low-lying coastal areas and deltas to the risks associated with sea level rise for many human and ecological systems, including increased saltwater intrusion, flooding and damage to infrastructure (high confidence). Risks associated with sea level rise are higher at 2°C compared to 1.5°C.
To the casual reader, these quotes might give the impression that raised Global Mean Surface Temperature is a direct cause of sea level rise. This assumption is wrong.
Radiation imbalance increases Earth’s surface temperature
Global warming is caused by human activity, which has made changes to the composition of the atmosphere and to the Earth’s surface. This is caused by a radiation imbalance: More radiant heat is entering the Earth than is leaving.
Due to this heating, Earth’s surface temperatures are rising. Some places, such as the Arctic, are having much faster rises than others.
Temperatures are measured at various places on Earth and these are combined to form an average figure called Global Mean Surface Temperature (GMST). Limiting the rise in this average figure, GMST, is at the centre of the Paris Agreement to “to pursue efforts to limit the temperature increase even further to 1.5 degrees Celsius”.
Radiation imbalance increases greenhouse heat in the Earth, especially in the oceans
Another effect of the radiation imbalance is to store extra heat in the Earth. Since it has been caused by the greenhouse effect, it can be called greenhouse heat. This greenhouse heat has been built up over the centuries since pre-industrial time.
It is usually measured in joules. GMST is measured in degrees centigrade.
Greenhouse heat and GMST are clearly different.
Where is the greenhouse heat?
Some greenhouse heat is stored at the surface of the Earth and in the atmosphere. This is a small part of greenhouse heat. The rest is stored in the oceans, as
- Ocean Heat Content (OHC),
- warmed sub-surface land,
- the latent heat of melted ice mass and thawed tundra.
More than 90% of the greenhouse heat is stored as OHC. IPCC AR5, WG1, Summary for Policymakers says:
Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010 (high confidence).
How is OHC related to GMST?
Raised sea surface temperatures pass heat through to lower layers by conduction and convection, the rate of this heat transfer being determined by surface temperature. However, radiant heat can by pass the surface penetrating the sea directly, to heat water beneath.
The surface temperature at a particular location is not the same as GMST, which is an average for all of Earth’s surface: e.g. Heat sinking into the Pacific is not directly affected by surface temperature in the Mongolian steppes.
It is clear that in referring to global warming that the two measures GMST and OHC are quite different and if one is used to stand in for the other it should be justified.
The Paris Agreement assumes GMST sidelining OHC
Methane and peak GMST
Methane in the atmosphere is the strongest Short-Lived Climate Pollutant (SLCP): Methane concentration in the atmosphere causes warming equal to 58% of that caused by CO2. This “methane forcing” causes a rise in Earth’s heat content.
However, methane has a short lifetime. It decays with half life of 9.1 years. After methane decays the component of the extra “methane heat”, that is stored at the Earth’s surface, is soon radiated to outer space leaving minimal effect on long term Global Mean Surface Temperature (GMST).
The Paris Agreement set the task of keeping the rise in GMST below or near to 1.5°C at the insistence of the Association of Small Island Developing States. This sets a target for a peak temperature, which will occur at a specific time in the future. (In an alternative formulation the target temperature is set for the year 2100, allowing GMST exceedance before this date. This too sets a specific date, i.e. 2100.)
Setting a date-specific limit to the averaged temperature, GMST, means that methane forcing that has occurred, say 20 years (about two half lives) before that date, has little effect on GMST at the chosen date, because of the rapid heat loss to outer space.
In short, if a date with a limit on Global Mean Surface Temperature is several decades in the future, current emissions of methane are of little importance. Any temperature rise they cause will dissipate before the target date.
Methane emissions and Global Mean Sea Level (GMSL)
The short-term effect of current methane concentrations on Global Mean Surface Temperature (GMST) does not apply to their effect on Global Mean Sea Level (GMSL): Average sea level acts differently to the averaged temperature, GMST.
The difference is because heat stored at the Earth’s surface dissipates much quicker than the heat stored in the rest of the Earth system: Surface layers cool in a few years; other heat stores, notably the ocean, cool over centuries. This is discussed in Centuries of thermal sea-level rise due to anthropogenic emissions of short-lived greenhouse gases, which says:
Our study shows that short-lived GHGs contribute to thermal expansion of the ocean over much longer time scales than their atmospheric lifetimes. Actions taken to reduce emissions of short-lived gases could mitigate centuries of additional future sea-level rise.
The long-term effect of Short Lived Climate Pollutants (SLCPs), especially methane, on sea level rise has not been made clear enough in IPCC reports.
It has been too easy to fall into the trap like this: Methane’s forcing has little consequence for keeping to the Paris temperature targets based so its forcing has little effect on sea levels. This is clearly false.
It is very likely that there is a substantial anthropogenic contribution to the global mean sea level rise since the 1970s. This is based on the high confidence in an anthropogenic influence on the two largest contributions to sea level rise, that is thermal expansion and glacier mass loss.
It should have been made clear that the “substantial anthropogenic contribution” included a considerable contribution from the forcing due to methane in the atmosphere.
Public and policy makers should be guarded from the false assumption that constraining global average temperatures to a given level solves the problem of rising sea levels.
Ocean heat and intense storms
A less recognised Cumulative Effect of methane emissions is the intensification of tropical storms to become super hurricanes and super typhoons (2011), caused by the cumulative increase in ocean heat content.
Recognition of the effects of increased ocean heat content is increasing as shown in Ocean heat as ‘fuel’ for hurricanes (2018):
Along with Lijing Cheng of China’s Institute of Atmospheric Research, Trenberth and Jacobs analyzed the source of the “fuel” for Hurricane Harvey, which deluged Houston and parts of Texas in August 2017.
“We’ve actually analyzed where the water vapor came from, and it came out of the ocean, and the ocean heat content was at record high levels,” Trenberth says in this month’s Yale Climate Connections “This Is Not Cool” video. “Water vapor is the fuel for all kinds of weather systems, whether it’s an individual thunder storm” or a hurricane or tropical cyclone.
Cheng offers that “Harvey extracted heat from the ocean and released heat by precipitation … so the ocean heat content losses were just exactly consistent with the energy of precipitation by Harvey.”
Negative carbon emissions, intense storms and sea level rise
Although the above concerns short-lived climate forcing by methane, similar arguments apply to CO2 when its lifetime is shortened – and CO2’s lifetime is shortened by “negative emission” or Carbon Dioxide Removal (CDR). CDR projects to reclaim CO2 from the atmosphere are present in various scenarios discussed by the IPCC.
Removing CO2 from the atmosphere at a date later than it was emitted does help Global Mean Surface Temperature targets. However, during the time between emissions and removals, heating from the CO2 will have raised Ocean Heat Content, created intense storms and raised sea levels, these effects will only diminishes slowly over centuries.
Time limited heating (e.g. from SLCPs or short-term CO2) causes feedbacks
A full examination of the effects of Short Lived Climate Pollutants (or short lived CO2) on climate feedbacks will not be discussed here, but IPCC SR15, Chapter 2 discusses some feedbacks that will release greenhouse gasses as the Earth warms:
Combined evidence from both models (MacDougall et al., 2015; Burke et al., 2017; Lowe and Bernie, 2018) and field studies (like Schädel et al., 2014; Schuur et al., 2015) shows high agreement that permafrost thawing will release both CO2 and CH4 as the Earth warms, amplifying global warming. This thawing could also release N2O (Voigt et al., 2017a, b). Field, laboratory and modelling studies estimate that the vulnerable fraction in permafrost is about 5–15% of the permafrost soil carbon (~5300–5600 GtCO 2 in Schuur et al., 2015)
Other reasons for worrying about methane emissions
Methane emissions should be cut but methane concentrations are rising:
- Increased importance of methane reduction for a 1.5 degree target
- Very Strong Atmospheric Methane Growth in the 4 Years 2014–2017: Implications for the Paris Agreement
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