The S&A products are intended to support informed discussion and decisions by policymakers, resource managers, stakeholders, the media, and the general public. They also help define and set the future direction and priorities of the program. The S&A products are also intended to meet the requirements of the Global Change Research Act of 1990. The law directs agencies to “produce information readily usable by policymakers attempting to formulate effective strategies for preventing, mitigating, and adapting to the effects of global change” and to undertake periodic scientific assessments.

Synthesis and Assessment Product 1.2: Past Climate Variability and Change in the Arctic and at High Latitudes

The US Geological Survey (USGS) led this assessment, which demonstrates for the first time the pervasive nature of Arctic climate amplification. Thirty-seven scientists from the United States, Germany, Canada, the United Kingdom and Denmark contributed to the report.

The report finds that temperature change in the Arctic is happening at a greater rate than other places in the Northern Hemisphere, and this is expected to continue in the future. As a result, glacier and ice-sheet melting, sea-ice retreat, coastal erosion and sea level rise can be expected to continue.

The new report also makes several conclusions about the Arctic:

• Taken together, the size and speed of the summer sea-ice loss over the last few decades is highly unusual compared to events from previous thousands of years, especially considering that changes in Earth’s orbit over this time have made sea-ice melting less, not more, likely.

• Sustained warming of at least a few degrees (more than approximately 4° to 13°F above average 20^th century values) is likely to be sufficient to cause the nearly complete, eventual disappearance of the Greenland ice sheet, which would raise sea level by several meters.

• The current rate of human-influenced Arctic warming is comparable to peak natural rates documented by reconstructions of past climates. However, some projections of future human-induced change exceed documented natural variability.

• The past tells us that when thresholds in the climate system are crossed, climate change can be very large and very fast. We cannot rule out that human induced climate change will trigger such events in the future.

By integrating research on the past 65 million years of climate change in the entire circum-Arctic, we have a better understanding on how climate change affects the Arctic and how those effects may impact the whole globe. This report provides the first comprehensive analysis of the real data we have on past climate conditions in the Arctic, with measurements from ice cores, sediments and other Earth materials that record temperature and other conditions.

—Mark Myers, USGS Director

Synthesis and Assessment Product 2.3: Atmospheric Aerosol Properties and Climate Impacts

Scientists need a more detailed understanding of how human-produced atmospheric particles, called aerosols, affect climate in order to produce better predictions of Earth’s future climate, according to this NASA-led report.

The study’s authors include scientists from NASA, the National Oceanic and Atmospheric Administration and the Department of Energy.

The influence of aerosols on climate is not yet adequately taken into account in our computer predictions of climate. An improved representation of aerosols in climate models is essential to more accurately predict the climate changes.

—Mian Chin, NASA Goddard Space Flight Center

Aerosols are suspended solid or liquid particles in the air that often are visible as dust, smoke and haze. Aerosols come from a variety of natural and human processes. On a global basis, the bulk of aerosols originate from natural sources, mainly sea salt, dust and wildfires. Human-produced aerosols arise primarily from a variety of combustion sources. They can be the dominant form of aerosol in and downwind of highly populated and industrialized regions, and in areas of intense agricultural burning.

Although Earth’s atmosphere consists primarily of gases, aerosols and clouds play significant roles in shaping conditions at the surface and in the lower atmosphere. Aerosols typically range in diameter from a few nanometers to a few tens of micrometers. They exhibit a wide range of compositions and shapes, but aerosols between 0.05 and 10 micrometers in diameter dominate aerosols’ direct interaction with sunlight. Aerosols also can produce changes in cloud properties and precipitation, which, in turn, affect climate.

Current predictions of how much Earth’s average surface temperature will increase in the future fall in a wide range. If the amount of carbon dioxide and other greenhouse gases double from the levels in the atmosphere in 1990, the increase in temperature is expected to be from 2.2 to 7.9 degrees Fahrenheit, according to the UN Intergovernmental Panel on Climate Change. The role of greenhouse gases in global warming is fairly well established, but the degree to which the cooling effect of human-produced aerosols offsets the warming is still inadequately understood. The report states that scientists should strive to improve their understanding of aerosols’ climate influences with the goal of cutting that range of uncertainty by nearly two-thirds.

The report states that to achieve the goal of reducing uncertainties in aerosol impacts on climate, an advanced, multi-disciplinary approach that integrates surface, aircraft, and space-based measurements with models will have to be developed. Scientists have made gains in modeling aerosol effects, but this capability has not yet been fully incorporated into climate simulations, according to the report.

The report advocates the development of new space-based, field, and laboratory instruments and the incorporation of more realistic simulations of aerosol, cloud, and atmospheric processes into climate models. The United States faces the challenge of maintaining and enhancing its existing aerosol monitoring capability from space. Satellites have been providing global aerosol observations since the late 1970s, with major improvements in accuracy since the late 1990s. But some of these missions, such as NASA’s suite of Earth Observing System satellites, are reaching or exceeding their design lives, the report notes.

Synthesis and Assessment Product 4.1: Coastal Sensitivity to Sea-Level Rise: A Focus on the Mid-Atlantic Region

This S&A report was led by the US Environmental Protection Agency (EPA) and examines multiple opportunities for governments and coastal communities to plan for and adapt to rising sea levels.

Global sea level is rising, and there is evidence that the rate is accelerating. Increasing atmospheric concentrations of greenhouse gases, primarily from human contributions, are very likely [>90% likelihood] warming the atmosphere and oceans. The warmer temperatures raise sea level by expanding ocean water, melting glaciers, and possibly increasing the rate at which ice sheets discharge ice and water into the oceans. Rising sea level and the potential for stronger storms pose an increasing threat to coastal cities, residential communities, infrastructure, beaches, wetlands, and ecosystems. The potential impacts to the United States extend across the entire country: ports provide gateways for transport of goods domestically and abroad; coastal resorts and beaches are central to the US economy; wetlands provide valuable ecosystem services such as water filtering and spawning grounds for commercially important fisheries. How people respond to sea-level rise in the coastal zone will have potentially large economic and environmental costs.

—Coastal Sensitivity to Sea-Level Rise

Sea-level rise can affect coastal communities and habitats in a variety of different ways, including submerging low-lying lands, eroding beaches, converting wetlands to open water, intensifying coastal flooding, and increasing the salinity of estuaries and freshwater aquifers. Some impacts of sea-level rise can already be observed along the US coast.

The primary causes of global sea-level rise are the expansion of ocean water due to warming and the melting of glaciers and ice sheets. Locally, sea-level rise is also influenced by changes to the geology of coastal land, making coastal elevation mapping an important area of future study. The Mid-Atlantic region, the focus of this report, is one of the areas in the US that will likely see the greatest impacts due to rising waters, coastal storms, and a high concentration of population along the coastline.

Highlights of the report include:

• Rising water levels are already an important factor in submerging low-lying lands, eroding beaches, converting wetlands to open water, and exacerbating coastal flooding.  All of these effects will be increased if the rate of sea-level rise accelerates in the future.

• Most coastal wetlands in the mid-Atlantic would be lost if sea level rises one meter in the next century.  Even a 50-cm rise would threaten most wetlands along the Eastern Shore of Chesapeake Bay.

• Possible responses to sea level rise include seawalls, bulkheads, and other shoreline armoring; elevating buildings and land surfaces (including beaches and wetlands); and allowing shorelines to change and moving structures out of harm’s way.  Those three approaches have very different environmental and social impacts.

• Preparing now can reduce the eventual environmental and economic impacts of sea level rise. Some governmental and nongovernmental organizations are already starting to prepare for sea level rise.

Synthesis and Assessment Product 4.2: Thresholds of Climate Change in Ecosystems

This report, also led by the USGS, found that light changes in climate may trigger major abrupt ecosystem responses that are not easily reversible. Some of these responses, including insect outbreaks, wildfire, and forest dieback, may adversely affect people as well as ecosystems and their plants and animals.

An ecological threshold is the point at which there is an abrupt change in an ecosystem that produces large, persistent and potentially irreversible changes.

One of our biggest concerns is that once an ecological threshold is crossed, the ecosystem in question will most likely not return to its previous state. The existence of thresholds should be a key concern of scientists and natural resource managers.

—Susan Haseltine, USGS Associate Director for Biology

The team also emphasized that human actions may increase an ecosystem’s potential for crossing ecological thresholds. For example, additional human use of water in a watershed experiencing drought could trigger basic changes in aquatic life that may not be reversible. Researchers and decision makers need to develop the tools necessary to predict the effects of specific ecological disturbances and to understand early warning signals of impending ecological thresholds.

The report also concludes that although not enough is known about ecological thresholds, researchers do already know that ecosystems will differ significantly in their respective thresholds. More vulnerable ecosystems, such as those that already face stressors other than climate change, will almost certainly reach their threshold for abrupt change sooner.

The Environmental Protection Agency, US Forest Service, Department of Energy, National Oceanic and Atmospheric Administration, and National Science Foundation contributed to this report.

Resources

• Past Climate Variability and Change in the Arctic and at High Latitudes (1.2)

• Atmospheric Aerosol Properties and Climate Impacts (2.3)

• Coastal Sensitivity to Sea Level Rise: A Focus on the Mid-Atlantic Region (4.1)

• Thresholds of Climate Change in Ecosystems (4.2)