Learning about Climate Change

Overall Trends in Climate Change

The climate is changing rapidly in the Northeastern United States (NEUS)  in ways that have already impacted fish, wildlife, and their habitats. These impacts will continue as climate change increases over the coming decades.
Warming is occurring in all seasons, with the greatest changes in winter, at higher latitudes, and potentially at higher elevations. Seasonal warming is extending the growing season, particularly with more frost free days occurring earlier in spring. Precipitation amounts are increasing, especially in summer and fall. Warmer winters are also resulting in more precipitation falling as rain instead of snow, leading to reduced snowpacks - though stronger blizzards may lead to locally higher snowpacks in some areas like New England. In the summer, heavier downpours combined with longer dry periods are expected, increasing the risk of both droughts and floods. Sea level is also rising at a rapid rate along coastlines, leading to coastal flooding, which is compounded by increasingly intense coastal storms, such as hurricanes.

• Temperature changes
• Precipitation changes
• Changes in hydrology
• Changes in winter
• Sea level rise
• Storms and floods
• Coastal Storms
• Drought
• Change in timing of seasons​

Climate changes over the past century can be explained through a combination of human and natural factors with the majority explained by human sources of emissions from burning fossil fuels. Even if all emissions ceased today, warming would continue for at least the next couple of decades. Therefore, it is important to explore ways to cope or adapt to these changes in addition to reducing or mitigating the effects of future warming. Local decisions made now in how natural resources are managed and conserved can also make important differences in the ability of fish and wildlife species to cope with future climate changes.

Warming temperature in the United States of America from 1750 - 2100, with projections after 2022. Data courtesy of Berkeley Earth, https://berkeleyearth.org/

Uncertainty

Climate change is an area of active research, and many uncertainties and gaps in our knowledge exist. As described below, there are many steps in projecting climate impacts through modeling approaches that can introduce sources of uncertainty ranging from the amounts of carbon emissions to be released over the coming decades, to differences in how climate models represent the Earth system, to unknowns about how fish and wildlife species will respond. While these uncertainties and knowledge gaps pose challenges for planning, they should not prevent decisions and actions. There are many things that are certain for the Northeast: the climate is warming, resulting in longer growing seasons, more extreme events, and many related impacts on wildlife and habitats (e.g., increased pests and disease, vegetation shifts). For these more certain aspects of climate change, plans and actions can be made with a high degree of confidence. For areas that are less certain, such as local scale precipitation and surface hydrology (e.g., terrestrial drought, river and stream flows, vernal pool formation), planners need to consider different management options and the available resources they have to deal with the full range of projected outcomes. Many decision-support approaches exist to guide actions in the face of uncertainty, including structured decision making and scenario planning.

Emission and Climate Scenarios

Storylines of Possible Futures are Developed From Emission Scenarios, Climate Scenarios, Model Variability and Uncertainty. Many aspects of future climate are uncertain. For instance, we do not know which policies or regulations will be enacted (if any) to help reduce greenhouse gas emissions. Nor do we know how technology and our culture will progress to decrease our dependence and use of fossil fuels. Our understanding of the Earth’s climate system is also imperfect. Our ability to predict the future is limited by the fact that our atmosphere cannot be perfectly represented by a set of equations (i.e., climate models). Because of these uncertainties, we cannot know how the future will look. However, we can know how the future might look. Therefore, future climate is described, not in terms of a single forecast, but as a range of possible conditions. Among that range, a few representative storylines, or “scenarios,” are examined (e.g., a worst-case scenario, a best-case scenario, and 1-2 mid-range scenarios). These storylines can be used to inform local adaptation strategies and actions related to the conservation and management of fish, wildlife and their habitats in the NEUS. 

• Emission scenarios describe future releases of greenhouse gases, aerosols, and other pollutants into the atmosphere, and are based on expected changes in human populations and technology.                
• Climate scenarios describe the average characteristics of a possible future climate (e.g. hotter and drier).                
• Overall, emissions scenarios are the driving force, or cause, while climate scenarios describe the effect.                

Because we do not know how humans will respond to climate change, more than one scenario or storyline is often used in future projections to portray a range of possible futures; for example, a high amount of climatic change would be depicted by a scenario in which emissions continue at current rates, and a low amount of change would assume policies and regulations reduce the emission of greenhouse gases.

RCPs vs SSPs: Complementary but Different

In modern climate assessments, two major scenario frameworks are used together:

Representative Concentration Pathways (RCPs)

RCPs describe greenhouse gas concentration trajectories over time and are labeled by the amount of radiative forcing (heat trapping) they produce by 2100 (e.g., 2.6, 4.5, 6.0, 8.5 W/m²).
Examples include:

• RCP 2.6 - very low forcing, assumes rapid emissions reductions                
• RCP 4.5 / 6.0 - intermediate stabilization pathways                
• RCP 8.5 - high forcing, often described as “business-as-usual”
   

RCPs do not describe how societies get there, only the climate forcing outcome.

Shared Socioeconomic Pathways (SSPs)

SSPs describe socioeconomic development pathways (e.g., policy choices, population growth, technology, inequality, fossil fuel dependency). They help answer how humans might arrive at certain emissions levels. Examples include:

• SSP1 - sustainability and low inequality                
• SSP2 - “middle-of-the-road” development                
• SSP3 - regional rivalry and high barriers to mitigation                
• SSP5 - fossil-fuel intensive growth    

SSPs therefore provide the narrative context, while RCPs provide the climate forcing outcome. In the most recent Intergovernmental Panel on Climate Change (IPCC) assessments, they are often used in combination (e.g., SSP1–2.6, SSP2–4.5, SSP5–8.5) to link socioeconomic storylines with emissions and climate projections.

Before RCPs and SSPs, climate studies used the Special Report on Emissions Scenarios (SRES) framework, which grouped scenarios into families (e.g., A1, A2, B1) based on assumptions about population, energy use, and economic growth. SRES scenarios are now largely historical and are mainly encountered in studies published prior to 2013.

It is important to emphasize that none of these scenarios carry probabilities, they are not predictions, but tools to explore a range of plausible futures. Using multiple scenarios allows researchers and managers to evaluate risks, plan adaptation strategies, and test how sensitive ecological systems are to different climate trajectories.

Climate Models

• Climate models are the tools that are used to study the impacts of greenhouse gas emissions on Earth’s climate.
• Emission scenarios are inputs into large scale models known as General Circulation Models or Global Climate Models (GCMs).
• GCMs capture physical processes of our global climate system including the atmosphere, ocean, ice sheet, and land surfaces.

Several climate models have been developed by different institutions around the world. Although these models were built from similar fundamental physical principles, they can predict different future conditions due to differences in how they represent more complex processes, such as land-atmosphere interactions. GCMs also have different spatial resolutions. Because different models can produce different results, it is customary to use an average across multiple models, known as an “ensemble average.” Over the last few decades, confidence in projections of future climate change using GCMs has increased as research has improved our understanding of large scale processes on Earth. 

Simulating our complex atmosphere with a climate model can require enormous computer resources and computation time. Global models, in particular, can take weeks to months to run on even the most advanced supercomputers. To save time and resources, global models are run at coarse resolution, and thus poorly represent local scale changes. Downscaling techniques are helpful in transforming the climate change information obtained from GCMs to much higher spatial resolution and can help us better understand local and regional scale climate changes. Similarly to GCMs, downscaling approaches can also yield different model results. While downscaling is a necessary step for adequately representing the local climate, the additional modeling introduces additional uncertainties due to differences in how models capture fine-scale atmospheric processes.

World Climate Research Program (WCRP) and Coupled Model Intercomparison Project (CMIP) are collections of model outputs that are freely available to the public. In 2008, CMIP5 were developed (extending upon CMIP3; there is no CMIP4) to serve as the basis for RCP4.5 and RCP8.5, the medium and high emissions scenarios, respectively.

In the short term (over the next 5-20 years), the direction and magnitude of warming in the global climate are mostly consistent across all emissions scenarios and with strong agreement across climate models. There is high certainty that the greater Northeast United States will continue to experience warming. It is likely that areas like New England will experience precipitation shifts from snow to rain, though shifts in the amount of total precipitation (rain and snow) are less certain. Severe weather events (e.g., thunderstorms, tornadoes) are challenging to project. Soil moisture and evapotranspiration (movement of water among plants, land, waterbodies, and atmosphere) trends are neither robustly observed nor consistent amongst modeling studies.

The Climate Adaptation Tool website draws heavily on the information in the “A regional synthesis of climate data to inform the 2025 State Wildlife Action Plans in the Northeast U.S.”(Staudinger et al 2024) which uses CMIP5, CMIP6 was constructed and released between 2015 and 2022. The latest CMIP6 climate models build on CMIP 5 by improving the representation of physical processes at dinner scales and expanding the range of future scenarios (Eyring et al., 2016; O’Neil et al., 2016; Stouffer et al., 2017). Unlike CMIP5, CMIP6 incorporates the Shared Socioeconomic Pathway (SSP) framework, which links climate forcing with different socioeconomic futures, including varying levels of mitigation and adaptation (Riahi et al., 2017; Hausfather, 2019). This allows researchers to better assess how societal responses interact with climate change.

Interpreting Climate Model Outputs

Interpretation of climate model outputs can be aided by a firm understanding of some important terms. Projections show a range of what could happen based on a range of future scenarios. In contrast, predictions describe what will happen assuming one particular scenario plays out. A forecast is a prediction used exclusively in predicting short-term (i.e., days to weeks) weather patterns. Model projections (i.e., what could happen) are not predictions (i.e., what will happen) because the final outcome depends on how greenhouse gas emissions change over time as policies and human activities shift.

Other Resources

• A regional synthesis of climate data to inform the 2025 State Wildlife Action Plans in the Northeast U.S.
• Designing Sustainable Landscapes Project
• Coastal Landscape Response to Sea-Level Rise Assessment for the Northeastern United States
• The Fifth National Climate Assessment
• NCDC. 2015. Climate at a Glance
• USGS National Climate Change Viewer
• University of Massachusetts Amherst Conservation Assessment and Prioritization System (CAPS) Program
• USGS

References

Staudinger, M.D., A.V. Karmalkar, K. Terwilliger, K. Burgio, A. Lubeck, H. Higgins, T. Rice, T.L. Morelli, A. D'Amato. 2024. A regional synthesis of climate data to inform the 2025 State Wildlife Action Plans in the Northeast U.S. DOI Northeast Climate Adaptation Science Center Cooperator Report. 406 p. https://doi.org/10.21429/t352-9q86

Hammond, J. S., R. L. Keeney, and H. Raiffa. 1999. Smart choices: A practical guide to making better decisions. Harvard Business Review Press, Cambridge, Massachusetts.

Hayhoe, K., C. P., Wake, T. G. Huntington, L. Luo, M. D. Schwartz, J. Sheffield, E. F. Wood, B. Anderson, J. Bradbury, A. DeGaetano, T. Troy, and D. Wolfe. 2006. Past and future changes in climate and hydrological indicators in the U.S. Northeast. Climate Dynamics 28:381-407.

Intergovernmental Panel on Climate Change (IPCC). 2007. Climate change 2007: impacts, adaptation, and vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. M. L. Parry, O. F. Canziani, J. P. Palutikof, P. J. van der Linden, and C. E. Hanson, editors. Cambridge University Press, Cambridge, UK.

Intergovernmental Panel on Climate Change (IPCC). 2013: Climate change 2013: the physical science basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Stocker, T.vF., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, and P.M. Midgley, editors. Cambridge University Press, Cambridge, UK.

Knutti R, and J. Sedláček. 2013. Robustness and uncertainties in the new CMIP5 climate model projections. Nature Climate Change 3:369-373.

Moss, R. H., et al. 2010. The next generation of scenarios for climate change research and assessment. Nature 463:747-756.

Rowland, E. L., M. S. Cross, and H. Hartmann. 2014. Considering Multiple Futures: Scenario Planning To Address Uncertainty in Natural Resource Conservation. US Fish and Wildlife Service, Washington, DC.

Walsh, J., D. Wuebbles, K. Hayhoe, J. Kossin, K. Kunkel, G. Stephens, P. Thorne, R. Vose, M. Wehner, J. Willis, D. Anderson, S. Doney, R. Feely, P. Hennon, V. Kharin, T. Knutson, F. Landerer, T. Lenton, J. Kennedy, and R. Somerville, 2014: Ch. 2: Our Changing Climate. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 19-67.

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