Temps de lecture :3 minutes
WHEN the UN Framework Convention on Climate Change came into force in 1994, climate change’s impacts seemed distant. Not any more. With daily reports of changes to glaciers, ice sheets, oceans and ecological systems, climate change seems upon us.
As a result, the debate over what to do is changing. Geoengineering schemes, once considered nearly science fiction, are now discussed seriously. Most attention, though, has focused on reducing emissions of carbon dioxide.
There is no question that to stop climate change in the long run requires a substantial reduction in CO2 emissions. However, significant opportunities exist to slow warming over the next few decades by reducing emissions of other greenhouse gases.
Only about half the warming that has occurred up to now is due to CO2. The rest is caused by other greenhouse gases, particularly methane (Proceedings of the National Academy of Sciences, vol 97, p 9875). Similarly, less than half of the total warming expected over the next 20 years will be caused by CO2. Methane, along with other gases such as carbon monoxide, volatile organic compounds (VOCs) and black carbon particles, will cause most of the changes.
Recent modelling shows the way to have the biggest impact on warming over this century is to immediately reduce emission of these gases, and keep them low (International Journal of Climate Change Strategies and Management, vol 1, p 42).
Methane is a much more powerful greenhouse gas than CO2. A tonne of methane is responsible for nearly 100 times more warming over the first five years of its lifetime in the atmosphere than a tonne of CO2. Methane is removed from the atmosphere much more rapidly than CO2, with a half-life of 8.5 years compared with many decades for CO2, but a tonne of methane eventually turns to 2.75 extra tonnes of CO2 in the atmosphere. Even without taking this into consideration, a tonne of methane emitted today will exert more annual warming than a tonne of CO2 emitted today until 2075. Not until the year 7300 will the cumulative warming exerted by the two become equal. It is truly carbon on steroids.
This makes methane a good target for emissions reductions. Indeed, methane’s shorter lifetime means that atmospheric levels are much more responsive to reduction by emissions cuts.
Another important consideration is the impact on human health. Of all the greenhouse gases, CO2 is one of the least damaging. Methane, on the other hand, is a precursor of ground-level ozone, which is a toxic air pollutant. Carbon monoxide, VOCs and black carbon particles are also directly harmful to human health.
The global health burden from these air pollutants exceeds that of any other environmental risk and even that of some major diseases, including malaria and TB. Cutting methane emissions and those of other health-damaging greenhouse pollutants would thus save many lives.
More serious attention to methane would also change the terms of climate change negotiations, possibly for the better. Taking methane into account would shift some of the burden of responsibility onto developing countries. It may seem unfair to make developing countries more accountable for warming than they are now. But on the other hand, a range of new opportunities would arise for them to participate in tackling global warming. They could therefore benefit from schemes that reward progress in reducing emissions.
Why, then, are methane and the other non-CO2 greenhouse gases not more prominent in discussions over global warming? One reason is that the official weighting scheme to assess the relative impacts of greenhouse gases is out of date and too focused on long-term warming.
According to this scheme, a tonne of methane is equivalent to 21 tonnes of CO2 over a 100-year period. This is out of date – current estimates put the ratio at 25 or more. More importantly, the timescale is all wrong given the urgency we now face. It gives equal weight to measures that will reduce warming in 2109 and warming next year. This is a rather odd perspective: surely reducing next year’s warming should be the priority.
Reducing methane levels in the atmosphere would arguably be less painful than reducing CO2. The technology already exists, and reductions would be politically and economically easier to implement. Methane is also easier to handle in international negotiations than black carbon, the next most important non-CO2 greenhouse pollutant, because its impacts are better understood.
Global methane emissions are divided roughly equally between the energy sector (coal mine emissions and leaks from oil and gas wells), waste management (landfill, waste water and animal manure) and agriculture (mainly rice paddies and emissions from livestock).
Reducing livestock and rice production would require changes in consumption, but that is not the case with waste handling and leaks from fossil-fuel systems. Fixes for these do not directly threaten lifestyles and are amenable to direct regulation; no need for controversial carbon taxes or cap-and-trade schemes.
We urgently need measures that can help keep Earth from overheating while we work to control CO2 emissions. Doing all we can to reduce methane emissions makes more sense than embarking on risky geoengineering schemes.
This fruit is low-hanging, ripe and heavy with immediate benefits. Helping to pick it also means I can tell my grandchildren that, yes, I did do something to directly protect the planet.
25 june 2009 by Kirk Smith, professor of global environmental health at the University of California, Berkeley
New Scientist Magazine n° 2714