The Fifth National Climate Assessment

This is the US Government’s preeminent report on climate change impacts, risks, and responses. It is a congressionally mandated interagency effort that provides the scientific foundation to support informed decision-making across the United States.

From the report:

• "Climate Is Changing, and Scientists Understand Why:  It is unequivocal that human activities have increased atmospheric levels of carbon dioxide and other greenhouse gases. It is also unequivocal that global average temperature has risen in response."

• "Earth’s climate is changing because humans are increasing the concentrations of greenhouse gases in the atmosphere by burning fossil fuels and through other activities. The increase in greenhouse gases is warming the planet and driving other observed climate trends, including increases in the frequency and severity of many types of extreme weather events. Future changes and impacts depend largely on the choices humans make about future greenhouse gas emissions."

• "The effects of human-caused climate change are already far-reaching and worsening across every region of the United States. Rapidly reducing greenhouse gas emissions can limit future warming and associated increases in many risks."

• Climate changes are making it harder to maintain safe homes and healthy families; reliable public services; a sustainable economy; thriving ecosystems, cultures, and traditions; and strong communities. Many of the extreme events and harmful impacts that people are already experiencing will worsen as warming increases and new risks emerge.

• With each additional increment of warming, the consequences of climate change increase. The faster and further the world cuts greenhouse gas emissions, the more future warming will be avoided, increasing the chances of limiting or avoiding harmful impacts to current and future generations.

• Large near-term cuts in greenhouse gas emissions are achievable through many currently available and cost-effective mitigation options. However, reaching net-zero emissions by midcentury cannot be achieved without exploring additional mitigation options. Even if the world decarbonizes rapidly, the Nation will continue to face climate impacts and risks. Adequately and equitably addressing these risks involves longer-term inclusive planning, investments in transformative adaptation, and mitigation approaches that consider equity and justice.

The Fourth National Climate Assessment (v1): Causes

This is the previous version (2017-2018) of the US's National Climate Assessment.

From the two-page Executive Summary Highlights:

"This assessment concludes, based on extensive evidence, that it is extremely likely that human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century. For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence."

The term "extremely likely" indicates a 95-100% level of confidence based on all the evidence.

The Fourth National Climate Assessment (v2): Impacts and Risks

The Overview provides a national summary and Chapter 18 is about impacts on the Northeast: https://nca2018.globalchange.gov/:

•  "Climate change creates new risks and exacerbates existing vulnerabilities in communities across the United States, presenting growing challenges to human health and safety, quality of life, and the rate of economic growth."

• "Climate change affects the natural, built, and social systems we rely on individually and through their connections to one another. These interconnected systems are increasingly vulnerable to cascading impacts that are often difficult to predict, threatening essential services within and beyond the Nation’s borders."

• "While mitigation and adaptation efforts have expanded substantially in the last four years, they do not yet approach the scale considered necessary to avoid substantial damages to the economy, environment, and human health over the coming decades."

• "The seasonality of the Northeast is central to the region’s sense of place and is an important driver of rural economies. Less distinct seasons with milder winter and earlier spring conditions are already altering ecosystems and environments in ways that adversely impact tourism, farming, and forestry."

• "The Northeast’s coast and ocean support commerce, tourism, and recreation that are important to the region’s economy and way of life. Warmer ocean temperatures, sea level rise, and ocean acidification threaten these services."

• "Changing climate threatens the health and well-being of people in the Northeast through more extreme weather, warmer temperatures, degradation of air and water quality, and sea level rise. These environmental changes are expected to lead to health-related impacts and costs, including additional deaths, emergency room visits and hospitalizations, and a lower quality of life."

The 2021 New Hampshire Climate Assessment

This report was supported by funding from the New Hampshire Department of Environmental Services and produced by NH State Climatologist M.D. Lemcke-Stampone, C.P. Wake, and E.A. Burakowski, from The Sustainability Institute, University of New Hampshire:  https://scholars.unh.edu/sustainability/71/.

•  "Temperatures across New Hampshire increased by an average of 3˚F since 1901. The rate of warming was greatest during the fall and winter seasons and at night. "

• "There was a 12% increase in annual precipitation over the past 120 years largely due to an increase in heavy precipitation since 1971. The number of multi-day precipitation events exceeding four inches increased near the coast and in the White Mountains while areas inland and in the far north saw an increase in the number of daily one-inch events.."

• "From 1971-2020, the amount of water stored in the snowpack decreased from 59-91% across central New Hampshire. Over the same period, spring ice-out dates on Lake Winnipesaukee and Lake Sunapee shifted eight and 11 days earlier, respectively.."