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A central finding of WGIII is that growth of income has been the largest single driver of emissions. Governments accepted that finding at the global level, where it is safe to discuss generalities because no country is in the spotlight. But WGIII also showed how different categories of countries contribute to global emissions (charts 1 to 3). We explain what was lost when these figures were cut from the SPM. Since the industrial revolution, today's highly industrialized countries have been the main contributors to greenhouse gas (GHG) emissions (chart 1). But over the past decade, their emissions have been roughly flat, whereas emissions from upper-middle-income countries (UMCs) have risen rapidly (chart 2A). The central implication is that inter-national climate policy needs to update how it categorizes countries. In the early 1990s when the UNFCCC was created, countries were divided into two categories—industrialized nations (Annex I), and the rest (non–Annex I). As non–Annex I countries' emissions have soared, their participation in future climate agreements is essential. Getting serious about categorizing countries will reopen old, but unavoidable, political fissures. Industrialized countries—which, on average, still have the highest per capita emissions—must do more to cut emissions. Emerging economies must accept that they account for most of the recent and expected emissions growth. Unless both groups take concerted action, stabilizing the climate will be impossible. There are many schemes for grouping countries. The best choice depends on the purpose of the analysis. The full WGIII report uses several schemes, but in the SPM a central purpose was to minimize variation in emissions within each category, and thus, it grouped countries according to underlying economic drivers of emissions. The writing team selected the latest version of a World Bank scheme widely used by experts in economic growth and better suited to explaining variations in emissions than the Annex I scheme. Nonetheless, there remains huge variation within these categories (chart 2B), particularly for the least-developed countries, where the greatest emissions are related to agriculture and land use. In richer countries it is energy consumption that drives most emissions. That key finding was also cut from the SPM. And any scheme is sensitive to the classification of China's big economy, which was upgraded to UMC status in 2010. Although income is an important driver (chart 2C), within income categories, many other factors, including policy, lead to highly variable emissions. Some UMCs have per capita emissions about equal to the most frugal industrialized countries whose income levels are a multiple. ![Figure][1] Historical view of CO2 emissions from fossil fuel combustion, flaring, cement, forestry, and other land use. Areas represent cumulative emissions. OECD, Organization for Economic Cooperation and Development. [Reproduced from figures TS.2a and 5.3 (1)] These figures also reveal the changing structure of the economy, measured in energy intensity (emissions per dollars of output, chart 2D). As economies mature, they become more efficient. But this improvement in energy intensity has not been fast enough to offset the overall effect of growing economies. That's why policies are needed and help explain the wide variation in emission intensity. Some countries have adopted aggressive policies, whereas others have done almost nothing. High variability in the factors that determine emissions has implications for the design of climate agreements. If a wider array of countries is to be engaged in the global effort to control emissions, a single schedule of binding targets and timetables is unlikely to work. International agreements must be flexible to accommodate different national circumstances, including uncertainty in economic development—a subject of much scientific research also cut from the SPM. Studies on effects of trade and globalization have tracked emissions “embodied” in products that are traded across borders. For the first time, IPCC presented adjusted emission statistics showing how territorial and consumption-based accounting systems lead to very different pictures. But because governments couldn't agree on how to group countries, all WGIII findings about embodied emissions were cut from the SPM. What was lost is shown in chart 3. Embodied carbon has increased dramatically since 1990. Current international policy approaches, which ignore trade, create strong incentives for countries to sit outside climate agreements and to take a free ride on benefits, not only in terms of climate change mitigation but also because of industry relocation. Better accounting would allow better incentives, such as border taxes and other adjustments for trade. Sophisticated rules within the World Trade Organization can keep countries from abusing such measures for protectionist purposes. Some countries are already experimenting with such approaches—the European Union (EU), for example, is extending its emission rules to cover foreign aircraft that use EU airports. Most talk of IPCC reform is focused on how it must change. For example, the IPCC should shift from infrequent big reports to more timely shorter and focused studies; the review process could be streamlined. But IPCC is a government-controlled process. Its line-by-line approval of the SPM yields the lowest common denominator of what is scientifically accurate and not too toxic for governments. A small number of countries can block findings that a large number of scientists working over many years with extensive review have agreed are robust. Disentangling IPCC from politics is impossible, especially where IPCC engages social science research that has policy-relevant conclusions. Yet IPCC as a scientific body can sharpen its messages by focusing more attention on the author-approved technical and policy summary documents. The most important rethinking is needed outside the IPCC process. When the IPCC began in the late 1980s, it eclipsed many different national climate science assessments. We predict and encourage a return to that multiplicity of assessments. This allows greater tailoring of assessments to local circumstances, values, and priorities. It will also encourage fuller diversity of scientific views. The IPCC will continue to have an important role to help stitch together many diverse assessments into a global perspective, while focusing on questions of special diplomatic importance such as impacts on the least-developed countries. [1]: pending:yes

For nearly three decades, the central goal in international climate policy had been to set the political agenda—to engage all countries on the need for action. So long as that was the goal, it was sufficient for policy-makers to focus on simple indicators of climate change, such as global average surface temperature With the 2015 Paris Agreement, governments launched a process that can move beyond setting agendas to coordinating national policies to manage the climate. Next month in Marrakesh, diplomats will convene to flesh out the Agreement. They need to focus on the infrastructure of data and analysis that will be needed as the Agreement becomes operational. The scientific community can help by identifying better lagging indicators to describe what has changed as policy efforts progress, and leading indicators to focus policy on the right risks as the planet warms.

Faced with government inaction, private firms emerge as major players in climate change mitigation

Climate change is a global phenomenon, but the institutions needed to implement effective policy reside mainly with national governments. This mismatch explains why serious efforts to control emissions of greenhouse gases, such as markets for emission credits, are fragmented across national and regional lines. Climate policy is emerging from the bottom up rather than through globally orchestrated treaties such as the Kyoto Protocol. The authors of this Policy Forum state that fuller efforts to control emissions will require serious engagement of the United States and developing countries, as well as viable schemes for integrating the many fragmented policies that are arising as governments grapple with the climate challenge.