Expected Effects in the U.S.

Temperature and Precipitation Projections

Global average temperatures are projected to rise over this century and beyond, causing continued changes in all components of the climate system. Temperature increases will vary regionally and seasonally; for example, temperature increases at polar latitudes are expected to be greater than increases near the equator (USGCRP 2014 Ch. 2). Part of this future warming is inevitable due to the long-lived greenhouse gases that are already present in Earth’s atmosphere. However the full extent of warming will depend in part on future emissions of greenhouse gases. The IPCC has developed  four ‘representative concentration pathways’, or RCP’s, that describe a plausible range of future emissions. To develop these RCP’s, scientists selected different levels of greenhouse gas concentrations for the year 2100 that are each possible given current scientific knowledge. Each pathway could be achieved by different socioeconomic scenarios, including future trajectories  of population, economic and technological change, and political choices  (IPCC 2013, Summary for Policy Makers). Having these RCP’s provides a way for  climate modelers to compare projected future climates using consistent sets of assumptions about future levels of emissions.  

By 2100, average temperatures in the U.S. are expected to increase by approximately 8°F or more (4.4°C) under a high RCP with similar rates to current greenhouse gas emissions and by approximately 2.5°F (1.4°C) under a  lower RCP that assumes immediate and rapid greenhouse gas reductions (USGCRP 2014 Ch. 2 – Figure 7). Both lower and higher temperature changes are possible, if future emissions fall below or above these pathways.

Figure 7 - Projected average temperature changes in the U.S. for 2071-2099, relative to the period from 1970-1999. The wide range in projections is due to the different pathways (RCP’s) that are considered. Source: USGCRP 2014 Ch. 2. -Third National Climate Change Assessment. 

Precipitation changes will also vary seasonally and regionally, and are more uncertain than temperature changes. Models project that northern areas in the U.S. will generally become wetter, and southern areas will generally become drier, especially the Southwest (USGCRP 2014 Ch. 2). In northern areas, a greater proportion of annual precipitation is expected in the winter and spring, and may fall as rain rather than snow due to warmer temperatures. In the Southwest,drier conditions are projected particularly for the winter and spring (USGCRP 2014 Ch. 2). Across all areas of the United States, the number of heavy precipitation has increased since the 1950’s, and is expected to increase further over the next century (Figure 8). Although modeled precipitation projections are improving and projected trends have remained consistent since the last IPCC report in 2007, there is still a high degree of uncertainty and specific regional patterns could differ from these general trends.

Figure 8 - Projected changes in the frequency of extreme daily precipitation events for 2081-2100, compared to 1981-2000. An extreme event is a daily amount of precipitation that now occurs once every 20 years. Under a low emissions pathway (RCP 2.6) these events would occur twice as often, under a high emissions pathway (RCP 8.5) events would occur as much as five times as often. Source: USGCRP 2014 Ch. 2. -Third National Climate Change Assessment. 

Effects on Ecosystems and Ecosystem Processes

The climate changes expected over the next century will have huge consequences for ecosystems and the benefits they provide, including the provision of wood and fuel, food, temperature and flood regulation, erosion control, recreational and aesthetic value, and species habitat, among others.

Climate changes are likely to affect important ecological processes that will in turn affect key natural resources. For example, temperature and precipitation changes have strong implications for water resources and hydrologic cycling. In addition, disturbances such as insects, wildfire, invasive plants, and forest diseases will become more frequent in some areas of the country. The emissions that cause climate change  also generate air pollution that can affect forest growth and health.

Coupled with altered hydrology and increased disturbance and stress, climate change will affect how species are distributed within the U.S., and will cause changes for aquatic ecosystems, wildlife species and soils. How these resources are affected will have broad implications for maintaining ecosystem services, including biodiversity and thecarbon storage capabilities of forests. Each impact on one aspect of an ecosystem can affect a variety of others, producing a series of cumulative effects that can make it difficult for ecosystems to adapt.

Meeting the diverse challenges that climate change presents for Earth's environments requires many approaches, and specific responses will depend heavily on the management goals for a particular resource see more at Managing Lands Under Climate Change. Scientists are currently working to understand the challenges posed to ecosystems by examining characteristics and changes in landscapes, modeling responses to climate change, and conducting assessments on impacts and ecosystem vulnerabilities Public lands, private lands, wilderness areas, and urban neighborhoods will all be affected, and each will require different management considerations. Specific management practices such assilviculture are potentially valuable tools for helping forests respond to a changing climate.

For those charged with managing ecosystems, climate change can seem like a daunting challenge. Fortunately, a range of management options exist to help ecosystems adapt to climate changes, and to contribute to climate change mitigation by reducing the amount of greenhouse gases in the atmosphere. These options are often complementary to actions that land managers employ regularly.

Need more information?

See the following primers and resources for more introductory information on climate change.

Climate Change Resource Center: 
FAQs

United States Global Change Research Program:
The Third National Climate Assessment

NASA Global Climate Change
Climate change: How do we know?

Center for Climate and Energy Solutions:
Climate Change – The Basics

Cooperative Institute for Research in Environmental Sciences:
Reading the IPCC Report - Recorded seminar series