Martin R. Manning

The next generation of scenarios for climate change research and assessment

Advances in the science and observation of climate change are providing a clearer understanding of the inherent variability of Earth’s climate system and its likely response to human and natural influences. The implications of climate change for the environment and society will depend not only on the response of the Earth system to changes in radiative forcings, but also on how humankind responds through changes in technology, economies, lifestyle and policy. Extensive uncertainties exist in future forcings of and responses to climate change, necessitating the use of scenarios of the future to explore the potential consequences of different response options. To date, such scenarios have not adequately examined crucial possibilities, such as climate change mitigation and adaptation, and have relied on research processes that slowed the exchange of information among physical, biological and social scientists. Here we describe a new process for creating plausible scenarios to investigate some of the most challenging and important questions about climate change confronting the global community.

The use of radiocarbon measurements in atmospheric studies.

(super 14) C measured in trace gases in clean air helps to determine the sources of such gases, their long-range transport in the atmosphere, and their exchange with other carbon cycle reservoirs. In order to separate sources, transport and exchange, it is necessary to interpret measurements using models of these processes. We present atmospheric 14CO (sub 2) measurements made in New Zealand since 1954 and at various Pacific Ocean sites for shorter periods. We analyze these for latitudinal and seasonal variation, the latter being consistent with a seasonally varying exchange rate between the stratosphere and troposphere. The observed seasonal cycle does not agree with that predicted by a zonally averaged global circulation model. We discuss recent accelerator mass spectrometry measurements of atmospheric 14CH (sub 4) and the problems involved in determining the fossil fuel methane source. Current data imply a fossil carbon contribution of ca 25%, and the major sources of uncertainty in this number are the uncertainty in the nuclear power source of 14CH (sub 4) , and in the measured value for delta (super 14) C in atmospheric methane.

Rising atmospheric methane: 2007-2014 growth and isotopic shift

From 2007 to 2013, the globally averaged mole fraction of methane in the atmosphere increased by 5.7 ± 1.2 ppb yr−1. Simultaneously, δ13CCH4 (a measure of the 13C/12C isotope ratio in methane) has shifted to significantly more negative values since 2007. Growth was extreme in 2014, at 12.5 ± 0.4 ppb, with a further shift to more negative values being observed at most latitudes. The isotopic evidence presented here suggests that the methane rise was dominated by significant increases in biogenic methane emissions, particularly in the tropics, for example, from expansion of tropical wetlands in years with strongly positive rainfall anomalies or emissions from increased agricultural sources such as ruminants and rice paddies. Changes in the removal rate of methane by the OH radical have not been seen in other tracers of atmospheric chemistry and do not appear to explain short-term variations in methane. Fossil fuel emissions may also have grown, but the sustained shift to more 13C-depleted values and its significant interannual variability, and the tropical and Southern Hemisphere loci of post-2007 growth, both indicate that fossil fuel emissions have not been the dominant factor driving the increase. A major cause of increased tropical wetland and tropical agricultural methane emissions, the likely major contributors to growth, may be their responses to meteorological change.

The trend in atmospheric methane δ13C and implications for isotopic constraints on the global methane budget

A recent paper by Tans [1997] has drawn attention to the isotopic disequilibrium that inevitably prevails when atmospheric methane is not in steady state with its sources, noting in particular the very slow adjustment of the isotopic signature δ13C toward its steady state. Our aim in this paper is to clarify the nature of disequilibrium effects on δ13C(CH4) and to assess their likely magnitudes in the global atmosphere over recent decades. We use a simple model simulation incorporating a plausible scenario of the global methane source history over 1700–2010, which includes an unchanged source since 1990. The simulation of both mixing ratio and δ13C compare favorably with the secular features of a 10-year data set (1988–1998) from Baring Head, New Zealand, and of a 17-year data set (1978–1995) in air archived from Cape Grim, Australia. This corroborates a recent analysis of those data sets and their compatibility with stabilized sources. We show that the slow adjustment of δ13C toward steady state arises from the effect of isotope fractionation on the cancellation of contributing terms to δ13C. We explore the implications of disequilibrium for the usual practice of relating δ13C values in the atmosphere to those in the aggregate source through a shift induced by fractionation and quantify the flaws in this practice. Finally, we examine the sensitivity of the atmospheric secular response, in both mixing ratio and δ13C, to sustained changes in source and sink and show that δ13C is a potentially powerful diagnostic of such changes.

Atmospheric carbon monoxide budget of the southern hemisphere: Implications of 13C/12C measurements

A large seasonal cycle has been observed in the 13C/12C isotopic ratio of CO in clean air in the extratropical southern hemisphere. This ratio is determined by the mixture of CO from isotopically distinct sources and is strongly influenced by the relative contributions of surface sources and the oxidation of CH4. We use a zonally averaged atmospheric model to relate atmospheric CO mixing ratios and 13C/12C isotopic ratios to the magnitude and distribution of CO sources and to explain the average seasonal cycles observed. The average 13C/12C ratio of CO emitted by surface sources in the southern hemisphere is larger than in the northern hemisphere and the southward flux of CO into the extratropical southern hemisphere is additionally enriched in 13C as a result of oxidation during transport. These effects partially offset the effect of highly depleted 13C/12C ratios in CO produced by CH4 oxidation. In the extratropical southern hemisphere, seasonal variation in the fraction of atmospheric CO derived from CH4 oxidation produces large changes in 13C/12 C which are partially offset by seasonal variations of surface sources. A good fit to observed average cycles can be obtained using surface source strengths consistent with previous estimates. However, the southern hemisphere data place strong constraints on the CH4 oxidation source and imply that either the CO yield per molecule of CH4 is about 0.7, compared with previous estimates of around 0.8, or that unidentified processes associated with CH4 oxidation cause 13C enrichment of about 4‰ in the CO produced.

Short-term variations in the oxidizing power of the atmosphere.

The hydroxyl radical is the predominant atmospheric oxidant, responsible for removing a wide range of trace gases, including greenhouse gases, from the atmosphere. Determination of trends and variability in hydroxyl radical concentrations is critical to understanding whether the ‘cleansing’ properties of the atmosphere are changing. The variability in hydroxyl radical concentrations on annual to monthly timescales, however, is difficult to quantify. Here we show records of carbon monoxide containing radiocarbon (14CO), which is oxidized by hydroxyl radicals, from clean-air sites at Baring Head, New Zealand, and Scott Base, Antarctica, spanning 13 years. Using a model study, we correct for known variations in production of 14CO (refs 6, 7), allowing us to exploit this species as a diagnostic for short term changes in hydroxyl radical concentrations. We find no significant long-term trend in hydroxyl radical concentrations but provide evidence for recurring short-term variations of around ten per cent persisting for a few months. We also find decreases in hydroxyl radical concentrations of up to 20 per cent, apparently triggered by the eruption of Mt Pinatubo in 1991 and by the occurrence of extensive fires in Indonesia in 1997.

Shipboard determinations of the distribution of 13C in atmospheric methane in the Pacific

Measurements of the mixing ratio and δ 13 C in methane (δ 13 CH 4 ) are reported from large, clean air samples collected every 2.5° to 5° of latitude on four voyages across the Pacific between New Zealand and the West Coast of the United States in 1996 and 1997. The data show that the interhemispheric gradient for δ 13 CH 4 was highly dependent on season and varied from 0.5‰ in November 1996 with an estimated annual mean of 0.2-0.3‰. The seasonal cycles in δ 13 CH 4 reveal three distinct latitude bands differentiated by phase. Maxima occur in January-February for the extratropical Southern Hemisphere, in September-October for the tropics, and in June-July for the extratropical Northern Hemisphere. The data are compared with results from a three-dimensional transport and atmospheric chemistry model that simulates the observed latitudinal structure of either δ 13 CH 4 or the methane mixing ratio well, but not both simultaneously. The requirement that a methane source-sink budget be consistent with both types of data clearly imposes stricter constraints than arise from either mixing ratio or isotopic data alone. The seasonal δ 13 CH 4 data in the extratropical Southern Hemisphere are used to estimate a value for the net fractionation in the CH 4 sink of 12-15‰, which is larger than can be explained by current laboratory measurements of a kinetic isotope effect for the OH + CH 4 reaction and soil sink processes. The hypothesis that the discrepancy is caused by competitive reaction of active chlorine with methane in the marine boundary layer is discussed.

Future changes in global warming potentials under representative concentration pathways

Global warming potentials (GWPs) are the metrics currently used to compare emissions of different greenhouse gases under the United Nations Framework Convention on Climate Change. Future changes in greenhouse gas concentrations will alter GWPs because the radiative efficiencies of marginal changes in CO2, CH4 and N2O depend on their background concentrations, the removal of CO2 is influenced by climate–carbon cycle feedbacks, and atmospheric residence times of CH4 and N2O also depend on ambient temperature and other environmental changes. We calculated the currently foreseeable future changes in the absolute GWP of CO2, which acts as the denominator for the calculation of all GWPs, and specifically the GWPs of CH4 and N2O, along four representative concentration pathways (RCPs) up to the year 2100. We find that the absolute GWP of CO2 decreases under all RCPs, although for longer time horizons this decrease is smaller than for short time horizons due to increased climate–carbon cycle feedbacks. The 100-year GWP of CH4 would increase up to 20% under the lowest RCP by 2100 but would decrease by up to 10% by mid-century under the highest RCP. The 100-year GWP of N2O would increase by more than 30% by 2100 under the highest RCP but would vary by less than 10% under other scenarios. These changes are not negligible but are mostly smaller than the changes that would result from choosing a different time horizon for GWPs, or from choosing altogether different metrics for comparing greenhouse gas emissions, such as global temperature change potentials.

Modeling the variation of δ13C in atmospheric methane: Phase ellipses and the kinetic isotope effect

We use the TM2 three-dimensional atmospheric tracer model with a methane source-sink budget based on existing literature to simulate small spatial and temporal variations in the 13C/12C ratio of atmospheric methane. The results show that δ13C varies markedly with wind direction everywhere outside the extratropical Southern Hemisphere (ETSH). Within the ETSH, both methane mixing ratio and δ13C have regular seasonal cycles with differing and latitude-dependent phases. Phase diagrams constructed from these seasonal cycles, showing changes in δ13C versus changes in mixing ratio, have elliptical shapes. The slope of the major axis of these ellipses is determined by the kinetic isotope effect (KIE) of the single atmospheric methane removal process used in the model. The ellipse eccentricity is determined by seasonal variation in the source δ13CH4, which is dominated by the biomass burning source because of its isotopic enrichment relative to other sources. Comparison of the model results, for a KIE based on CH4 + OH oxidation, with observations in the South Pacific region shows significant discrepancies in both the ellipse major axis slopes and eccentricities. We suggest that this is an indicator of an additional sink process that discriminates strongly against 13CH4. Such a sink could be active chlorine in the marine boundary layer.

Broader perspectives for comparing different greenhouse gases

Over the last 20 years, different greenhouse gases have been compared, in the context of climate change, primarily through the concept of global warming potentials (GWPs). This considers the climate forcing caused by pulse emissions and integrated over a fixed time horizon. Recent studies have shown that uncertainties in GWP values are significantly larger than previously thought and, while past literature in this area has raised alternative means of comparison, there is not yet any clear alternative. We propose that a broader framework for comparing greenhouse gases has become necessary and that this cannot be addressed by using simple fixed exchange rates. From a policy perspective, the framework needs to be clearly aligned with the goal of climate stabilization, and we show that comparisons between gases can be better addressed in this context by the forcing equivalence index (FEI). From a science perspective, a framework for comparing greenhouse gases should also consider the full range of processes that affect atmospheric composition and how these may alter for climate stabilization at different levels. We cover a basis for a broader approach to comparing greenhouse gases by summarizing the uncertainties in GWPs, linking those to uncertainties in the FEIs consistent with stabilization, and then to a framework for addressing uncertainties in the corresponding biogeochemical processes.