1. Introduction
1.3 Report structure
There are significant challenges in acquiring and presenting an integrated understanding of the impact of changing methane emissions on Arctic climate. The approach taken in this report is to first present the group of chapters that deal with natural
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Temperature anomaly, °C
2010 2000
1950 1960 1970 1980 1990
Mean 1961-1990
GISS MLOST HadCRUT4
Fig. 1.2 Annual average combined land and sea-surface temperature anomalies, 1950–2012 (relative to the mean over 1961–1990) for the area north of 60°N for three data sets: the Hadley Centre and Climate Research Unit dataset (HadCRUT4), the NASA Goddard Institute for Space Studies dataset (GISS), and the National Atmospheric and Oceanic Administration merged land-ocean surface temperature dataset (MLOST).
Trends (mean and 90% confidence intervals) over the 63-year period are 1.56°C (1.06–2.10°C) HadCRUT4; 1.73°C (1.15–2.32°C) GISS; and 1.46°C (1.06–1.83) MLOST.
4 This temperature analysis is based upon the Hadley Centre and Climate Research Unit dataset (HadCRUT4), the NASA Goddard Institute for Space Studies dataset (GISS), and the National Atmospheric and Oceanic Administration merged land-ocean surface temperature dataset (MLOST). Temperature anomalies in the three datasets are relative to different base periods. They are adjusted to a common base period 1961–1990 for time series plotting. The linear trend was computed using Mann-Kendall in combination with the Theil-Sen approach following Wang and Swail (2001) to account for auto-correlation in the time series.
For robust trend analysis, selection criteria were applied such that only sites with at least 50 years of data over the 63-year period, and at least six months of each year or two months of each season are used to compute annual or seasonal averages. Environment Canada, Climate Research Division, August, 2014.
0 0.5 1 1.25 1.5 1.75 2 2.5 3 4
HadCRUT4 annual GISS annual
GISS summer GISS winter
Observed temperature change, °C NOAA MLOST annual
Fig. 1.3 Observed Arctic warming, 1950–
2012, over the regions north of 60°N.
For the Hadley Centre and Climate Research Unit dataset (HadCRUT4) and the National Atmospheric and Oceanic Administration merged land-ocean surface temperature dataset (MLOST), temperatures are averaged over 5°×5°
grid boxes. The NASA Goddard Institute for Space Studies (GISS) data are averaged over 2°×2° grid boxes.
Projected temperature change, °C RCP2.6 Summer (JJA)
RCP8.5 Winter (DJF) RCP2.6 Winter (DJF)
RCP8.5 Summer (JJA)
Fig. 1.4 Projected change in surface air temperature over high-latitude areas for the period 2081–2100 relative to the reference period 1986–2005. Upper panels are median responses from 32 global climate models based on the RCP2.6 scenario, over the winter (DJF) and summer (JJA) seasons. Lower panels are based on 39 models using the RCP8.5 scenario. Adapted from IPCC Working Group I Fifth Assessment Report Annex I Supplemental Information (A1.SM2.6.21 and.23, and A1.SM8.5.21 and .23) (IPCC 2013c).
and anthropogenic methane sources, then present chapters that address atmospheric concentrations (as the atmosphere integrates emissions from all sources) and finally to present the results of modeling work that explicitly evaluates how changing sources will influence atmospheric concentrations and climate. As per the mandate to the Methane Expert Group, the scope of analysis is primarily Arctic-focused. While some AMAP assessments use a delineation of the Arctic region as defined by AMAP (land and marine areas north of the Arctic Circle, and north of 62°N in Asia and 60°N in North America, modified to include the marine areas north of the Aleutian chain, Hudson Bay, and parts of the North Atlantic Ocean including the Labrador Sea; AMAP 2011a, Fig. 1.1), for this assessment, no specific definition of an Arctic boundary was assumed and each chapter articulates boundaries suitable to the analysis within that chapter. Readers should note that while many of the chapters focus on the Arctic as a northern latitude region, where there is discussion of anthropogenic emissions of methane, the perspective is of Arctic nations as political entities, including all areas within their national borders. Given that methane is a global greenhouse gas, the technical chapters begin with an overview of the global methane budget (sources and sinks), and the role of methane as a greenhouse gas and climate forcer (Ch. 2). This provides essential background scientific information as well as the global context for understanding the subsequent chapters of the report, which are more Arctic focused. As Ch. 2 is provided for context, no key findings or conclusions are provided for that chapter.
Chapters 3 and 4 summarize current understanding of the natural processes that produce methane in Arctic environments and that may lead to enhanced emissions of methane from major terrestrial (Ch. 3) and marine (Ch. 4) sources. This work assesses the available published literature on these topics, drawing on both observational studies using flux measurements of methane to the atmosphere, and modeling studies. Emissions of methane from human activity are also changing and may also be contributing to recent changes in atmospheric methane levels. An assessment of available global methane emissions inventories is provided in Ch. 5 along with information specific to Arctic nations. In addition, Ch. 5 presents two scenarios of potential future anthropogenic methane emissions. One assumes no additional methane mitigation beyond existing legislation; the other is based on maximum technically feasible emission reductions with current abatement technologies. This scenario is global, but information specific to Arctic nations is extracted from the scenario.
Chapters 6 and 7 address the issue of how atmospheric concentrations of methane respond to changing emissions.
Chapter 6 presents an overview of the current atmospheric methane monitoring network over the Arctic region. Data from these sites are then analyzed and combined with previously published information to characterize trends and changes in atmospheric methane levels over time, on seasonal and longer time scales. Isotopic and trajectory analyses are explored as potential tools for detecting changes in methane emissions Fig. 1.5 The location of the zero degree near-surface air temperature isotherm for the historical 1996–
2005 period and the future 2081–
2100 period for the RCP2.6 and RCP8.5 scenarios. The zero degree isotherm is based on ensemble-mean annual air temperature simulated by 29 models (see below) that participated in the CMIP5 and averaged over the 10-year period centered on 2000 and 2090. Map created by Environment Canada’s Climate Research Division, December, 2014.
The 29 models from which results are used are BNU-ESM, CCSM4, CAM5, CESM1-WACCM, CNRM-CM5, CSIRO-Mk3-6-0, CanESM2, EC-EARTH, FGOALS-g2, FIO-ESM, CM3, ESM2G, GFDL-ESM2M, GISS-E2-H, GISS-E2-R, HadGEM2-AO, HadGEM2-ES, LR, IPSL-CM5A-MR, MIROC-ESM, MIROC-ESM-CHEM, MIROC5, MPI-ESM-LR, MPI-ESM-MR, MRI-CGCM3, NorESM1-M, NorESM1-ME, bcc-csm1-1, and bcc-csm1-1-m.
0
0 0
Position of 0°C near-surface air temperature isotherm Historical (1996-2005)
RCP2.6 (2081-2100) RCP8.5 (2081-2100)
What is the potential benefit, in terms of reduced Arctic warming, of methane emissions mitigation by Arctic nations?
How does the magnitude of potential emission reductions from anthropogenic sources compare to potential changes in methane emissions from natural sources in the Arctic?
Chapters 3 & 4 What are the current and potential future natural emissions from the Arctic region?
Chapter 5
What are the current and potential future anthropogenic emissions of Arctic and non-Arctic nations?
Chapters 6 & 7
Are the current monitoring activities (of atmospheric concentrations and fluxes) sufficient to capture anticipated source changes?
Chapter 8
What is the historical and future Arctic climate response to changes in methane emissions, from Arctic and from global sources?
Various chapters What are the uncertainties in understanding the Arctic climate response to methane?
What are the current methane emissions from Arctic terrestrial and marine sources?
What are the controlling processes and factors that strongly influence natural emissions?
How may these emissions from natural sources in the Arctic change in the future?
What are the uncertainties or limitations in these estimates?
What are current global anthropogenic methane emissions, and those of Arctic nations?
How will the magnitude of emissions change in the future under different policy assumptions?
What percentage of global methane mitigation potential is controlled by Arctic Council nations?
What are the principal sources of uncertainty in these estimates of current and future anthropogenic emissions?
What are the trends and variability in Arctic methane concentrations and what are the primary drivers of this variability?
How much of a trend in atmospheric methane abundance can be detected with the current monitoring network?
Are emission estimates consistent with atmospheric concentrations?
Is there evidence of increasing Arctic methane emissions in the atmospheric observations?
What is the contribution of historical changes in global atmospheric methane to Arctic climate warming?
What impact will increasing atmospheric concentrations of methane have on climate and will Arctic nations have the ability to influence that impact through mitigation of anthropogenic methane emissions?
How will atmospheric methane concentrations change in response to potential changes in natural methane emissions and how do these changes compare to those that might result from mitigation of anthropogenic methane emissions?
Does the location of anthropogenic methane emissions matter?
Table 1.1 Policy-relevant science questions guiding the work of the AMAP Methane Expert Group.
from different sources. Chapter 7 more explicitly integrates information on both atmospheric methane levels and emissions by taking a top-down inverse modeling approach to assess the extent to which changes in atmospheric methane in the Arctic can be explained and reconciled with estimates of natural and anthropogenic emissions in the Arctic. This information is also used to assess whether or not the atmospheric observations provide any indication of a trend in Arctic methane emissions.
In Ch. 8, the importance of past and potential future changes in methane emissions or concentrations on Arctic climate are discussed. In particular, the results of dedicated climate modeling experiments using the emission estimates from Ch. 3, 4, and 5 are presented, aimed at answering the overarching questions posed to the Methane Expert Group. Earth System Models are used to evaluate the benefit of anthropogenic methane emissions abatement in terms of reduced global and Arctic warming. Scenarios of natural emission change, founded on the analyses in Ch. 3 and 4, are used to calculate resulting changes in atmospheric methane concentration, allowing an estimate of warming resulting from such changes.
Each of the chapters in this report address a number of more specific policy-relevant science questions than the overarching questions presented at the end of Sect. 1.1. As a guide to the scope of work undertaken as part of this assessment, and to where readers can find information of particular interest, these questions are presented in Table 1.1. Key findings that respond to the questions in Table 1.1 are presented at the end of each chapter, along with recommendations for ongoing scientific work needed to address gaps in understanding. Chapter 9 presents a synthesis of these key findings and science recommendations.
The report concludes with a detailed summary of the strategies used for modelling the climate response. This annex is a common
contribution to the AMAP assessments on methane (the present report) and black carbon and ozone (AMAP 2015) and has been produced to facilitate an integrated understanding of the separate climate modelling exercises undertaken by the two AMAP expert groups on short-lived climate forcers (SLCFs).
Acknowledgments
The authors of this chapter are grateful for valuable comments on a draft of this chapter from John Walsh, University of Alaska Fairbanks, and from anonymous external reviewers. We are also grateful to Guilong Li, of Environment Canada’s Climate Research Division for his contributions to the development of Figs. 1.2 and 1.3.