Many factors contribute to variability in Earth’s climate on a range of timescales, from seasons to decades. Natural climate variability arises from two different sources: (1) internal variability from interactions among components of the climate system, for example, between the ocean and the atmosphere, and (2) natural external forcings, such as variations in the amount of radiation from the Sun. External forcings on the climate system also arise from some human activities, such as the emission of greenhouse gases (GHGs) and aerosols. The climate that we experience is a combination of all of these factors.
Understanding climate variability on the decadal timescale is important to decision-making. Planners and policy makers want information about decadal variability in order to make decisions in a range of sectors, including for infrastructure, water resources, agriculture, and energy.
In September 2015, the Board on Atmospheric Sciences and Climate and the Ocean Studies Board of the National Academies of Sciences, Engineering, and Medicine convened a workshop1 (Statement of Task in Appendix A) to examine variability in Earth’s climate on decadal timescales, defined as 10 to 30 years. During the workshop, ocean and climate scientists reviewed the state of the science of decadal climate variability and its relationship to rates of human-caused global warming, and they explored opportunities for improvement in modeling and observations and assessing knowledge gaps. This report summarizes the workshop presentations and discussions. As such, it is a snapshot of how leading U. S. scientists were approaching the topic at the time. This report does not attempt to provide a complete overview of this rapidly advancing field or present any work not discussed at the workshop or any new work published since the workshop.
The scientific community broadly agrees that the planet as a whole is warming steadily through time (IPCC, 2014). Many workshop participants acknowledged that climate variability can cause the rate of warming to shift over periods lasting from years to a few decades. Internal climate variability can result from shifts in the absorption and transport of heat into the ocean, leading to periods when Earth’s surface warms more slowly or more rapidly. Key points from workshop participants for framing the discussions ahead are highlighted in Box 1.
Since 1880, the average temperature at Earth’s surface has increased by about 0.85 C. Most of this increase (about 0.72 C) has occurred since 1951 (Hartmann et al., 2013). A number of recent studies indicate that the global mean surface warming trend slowed to near zero (0.07±0.08 C per decade) in the first to second decades of the 21st century (Easterling and Wehner, 2009; Hartmann et al., 2013; Kosaka and Xie, 2013) in relation to the trend during the latter half of the 20th century (i. e., 1950-2012). This slowdown in GMST rise has spurred much research aimed at examining recent and past climate variability in order to understand and better predict decadal climate trends. This period, typically defined as a range between 1998 to 2014, is referred to throughout this report as the “slowdown.” The scientific community, the media, and some workshop participants have also broadly used
1 This report has been prepared by the workshop rapporteur as a factual summary of what occurred at the workshop. The planning committee’s role was limited to planning and convening the workshop. The views contained in the report are those of individual workshop participants and do not necessarily represent the views of all workshop participants, the planning committee, or the National Academies of Sciences, Engineering, and Medicine.
the terms “hiatus” and “warming pause,” but through the discussions, many participants agreed that “slowdown” is a more accurate term because it does not suggest that something, specifically human-caused climate change, halted during this period (see Box 1, bullet 4).
A major line of inquiry discussed at the workshop is the degree to which natural variability modulated human-caused climate change during the recent warming slowdown, as well as during past periods of increased or accelerated warming, such as from 1970 to 1998. Also discussed was the extent to which previous results are a function of data coverage or remaining biases in sea surface temperature (SST) reconstructions. Some research has indicated that the early-2000s warming slowdown does not appear to be as pronounced if incomplete observed data coverage over the Arctic or errors in calibration of SST observations are taken into account (e. g., Cowtan and Way, 2014; Karl et al., 2015).
Much of the workshop discussion focused on the mechanisms governing decadal variability. Several participants presented evidence that the recent slowdown is driven in large part by well-documented swings in Pacific SSTs and sea level pressure known as the Interdecadal Pacific Oscillation (IPO). Other research has made the case that external forcing also played a role; for example, multiple small- to moderate- sized volcanoes have produced an accumulation of aerosols in the stratosphere that contribute to cooling (e. g., Ridley et al., 2014; Santer et al., 2014).
The specific mechanisms driving decadal variability, not only in the Pacific but also in all of the ocean basins, are subjects of intense scientific inquiry. Workshop participants shared research into potential mechanisms driving Pacific temperature swings, including storage of excess heat in the deeper ocean, movement of heat to the Indian Ocean, wind-driven changes, and teleconnections with the Atlantic Ocean. Proposed mechanisms of Atlantic variability include changes induced by the ocean’s major current (the Atlantic Meridional Overturning Circulation, or AMOC) and its relationship to the North Atlantic Oscillation (NAO). Also discussed was variability in the Indian Ocean and polar regions.
Because the storage of heat in the ocean has been implicated in the recent warming slowdown as measured by GMST, participants discussed the limitations of using GMST as the primary metric of global climate change. Many participants supported the notion that, because 93 percent of the excess heat from GHGs is stored in the ocean, sea-level rise, or sea-level rise together with GMST, may be a more appropriate metric of global climate change.
Variability at decadal timescales is a well-known feature of the climate system. Climate models produce periods of slower and more accelerated warming, although a specific slowdown in the GMST warming trend in the early 2000s was not directly projected by climate models (the warming trend during this period was near the lower edge of the 5-95 percent range of projections from the Coupled Model Intercomparison Project Phase 5; Schmidt et al., 2014). Many participants agreed that being able to predict decadal variability would be important given its implications, for example, its link to important regional phenomenon such as drought. Much remains to be learned before scientists will be able to make skillful predictions of variability on these timescales, however.
Workshop participants discussed the importance of advancing understanding of how all of the physical mechanisms in the ocean and atmosphere work in concert to produce decadal variability in the GMST and of improving observations and modeling capabilities in order to make predictions. Participants identified the continuation and improvement of ocean and atmospheric monitoring as well as more creative ways to use existing data, including paleoclimate data and synthesis products from models, as possible opportunities to improve predictions.
The study of Earth’s climate system involves a large and diverse group of experts. Participants commented on the great value provided by bringing together a diversity of researchers to discuss key challenges and opportunities in the field of decadal climate variability.