At the international climate negotiations in Paris last December, nearly all of the nations of the world, developed and developing, agreed to a “pledge and review” approach to reducing emissions of carbon dioxide and other greenhouse gases. Each of the participating countries thereby pledges a voluntary emissions reduction, usually expressed as a percentage decrease in total national emissions between a date in the past and a target year in the future. The Paris agreement thus aims to hold the global average temperature increase to well below 2 degrees Celsius above pre-industrial levels, and to attempt to further limit the increase to 1.5 degrees. The agreement also requires that the parties take global stock of their pledges at five-year intervals and update and enhance the pledges as needed to meet the stated temperature increase limits.
Much has been written about the political realities that resulted in this voluntary and non-binding approach, and whether it is likely to work. Indeed, the pledges that have been made so far already seem unlikely to meet the 2-degree target, based on current climate models. Nevertheless, there is good news: Nearly all the nations on Earth are now engaged in the process of limiting greenhouse gas emissions and are pledged to report their emissions.
But how will we know whether the emissions they report are correct, and whether their pledges are being fulfilled? Nearly all climate legislation—from the pioneering United Nations Framework Convention on Climate Change (UNFCCC) of 1992 onward—requires “bottom-up” reporting of emissions. Sources of greenhouse gas emissions are inventoried and multiplied by an emission factor to yield a national emission rate for each source and each greenhouse gas. Emissions of gases other than carbon dioxide are, by convention, multiplied by the Global Warming Potential, which relates the radiative efficiency as a warming agent of each gas to that of carbon dioxide, over a 100-year time horizon. National emissions on a carbon-dioxide-equivalent basis are then reported as the sum of these quantities for all reported greenhouse gases.
Unfortunately, atmospheric scientists have known for many years that emissions reported in this way can differ surprisingly from the emissions determined by measuring the actual increases of emitted gases in the atmosphere, so-called “top-down” emissions verification. For the global emission of long-lived gases this is a relatively easy problem to quantify. Early in the pioneering research on the global increase of carbon dioxide by Charles Keeling, it was already clear that the global combustion of fossil fuels, plus the net global deforestation reported by terrestrial ecologists, minus the uptake of carbon dioxide by the oceans, over-predicted the measured increase of carbon dioxide in the atmosphere. The discrepancy turned out to be due mainly to errors in assessing changes in the amount of carbon in the terrestrial biosphere, which at that time represented a net global sink for atmospheric carbon dioxide, despite massive deforestation in some regions. More recently, reported global emissions of the very potent and long-lived industrial greenhouse gases sulfur hexafluoride, carbon tetrafluoride, and nitrogen trifluoride have under-predicted their actual rates of increase in the global atmosphere, each by at least a factor of two.
Although the countries of the world share the global atmosphere, emissions reporting and reduction commitments under the Paris agreement and other regulatory legislation are divided among political entities that currently rely upon bottom-up emissions reporting processes that are known to be inaccurate. To effectively use top-down emissions verification to reduce these inaccuracies, atmospheric measurements and modeling must be carried out in a way that applies to emissions within national or regional boundaries.
Top-down emissions verification within geographic boundaries is an evolving area of research in which there has been great progress in recent years. The approach involves measuring atmospheric composition changes with high frequency and at several locations across a particular region; coupling these measurements with regional air transport and mixing models based on re-analysis of weather observations; and producing maps of integrated greenhouse gas emissions by nation or region. For example, a network of stations across Europe has been used to assess emissions of methane by individual nations, and in some cases to actually revise emission factors to bring bottom-up reported national emissions into better agreement with atmospheric observations. In another example, aircraft measurements over California in 2010, coupled with models of atmospheric transport, showed that statewide emissions of methane were roughly twice the bottom-up values compiled by the California Air Resources Board. And in a striking example of international verification and compliance, atmospheric measurements of the potent industrial greenhouse gas HFC-23 at Japanese and Korean monitoring stations have been coupled with atmospheric models to identify the locations of facilities emitting this gas in China, and to confirm that these emissions were effectively reduced under the Clean Development Mechanism of the Kyoto Protocol.
During the Cold War, President Reagan famously quoted the Russian proverb “trust, but verify” in the context of nuclear disarmament. Toward this end, the Comprehensive Test Ban Treaty Organization now monitors atmospheric radionuclides in a network of 80 stations around the globe. Using atmospheric transport models similar to those used to quantify greenhouse gas emissions, the goal of this network is to identify the location and nature of nuclear processing and testing activities, and thus to monitor and verify treaty compliance.
But despite the importance of the climate problem and its potential impacts on mankind, no adequately dense and coordinated global network exists for the measurement and modeling of greenhouse gas emissions. Now—under the Paris agreement with its “pledge and review” approach, and given the large uncertainties in current bottom-up emissions reporting procedures—it is past time to establish such a greenhouse gas measurement network. Modeling can always be improved, but measurements cannot be made retrospectively. Failure to regulate greenhouse gas emissions without such independent verification would be like going on a diet without weighing oneself.