Sea level, methane and a false assumption
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 problematic, particularly if it is widespread, because it is false.
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. (At least one place, a spot in the North Atlantic, is registering a fall in surface temperature.)
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”.
In brief, the Earth’s radiation imbalance causes a rise in GMST, a measure of globally averaged temperatures .
Radiation imbalance increases the heat in the Earth, especially in the oceans
Another effect of the radiation imbalance is to store extra heat in the Earth. Some of this extra heat is stored on or near the surface of the Earth, raising surface temperatures (and the globally averaged temperature, GMST). However, this is a small part of the increased heat. About 90% of this extra heat is stored in the oceans, as IPCC AR5, WG1, Summary for Policymakers pointed out:
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).
Local temperature increases may increase the Ocean Heat Content (OHC). However, it is problematical to consider that heat sinking into the Pacific is directly affected by the contemporaneous surface temperature in Mongolia.
Radiant heat can also penetrate the sea directly, bypassing local surface temperatures, and when radiation falls on ice masses, temperature increases can be stifled by the latent heat associated with melting, so the causal trail does not wholly pass through the route of increased local temperatures.
The globally averaged measure of temperature, GMST, is not a good candidate for “the cause” of the the increase in the heat stored in the Earth.
To link OHC to GMST can be a causality trap
To summarise, the Earth’s radiation imbalance causes a rise in Earth’s heat content, mostly stored in the oceans as Ocean Heat Content, but any assumed direct causal link between Global Mean Surface Temperature and Ocean Heat Content is misleading: They are both consequences of Earth’s radiation imbalance. It is misleading to assume that one causes the other.
An assumption that OHC is directly determined by GMST is a causality trap.
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 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.
Negative carbon emissions 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 and sea levels, an effect that 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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