Title: Fourth National Climate Assessment, Chapter 2: Our Changing Climate

Agency: U.S. Global Change Research Program

Comments Close: January 31st, 2018

Summary Index: 

  1. Purpose
  2. Context: The National Climate Assessment & US Global Change Research Program; Chapter 2 Writing Process; Tips for Chapter Review
  3. Details: Key Messages; Summary of 3 Select Key Messages (chosen by Contributor)


Source: Mark Stevens, Creative Commons


Volume II of the Fourth National Climate Assessment “summarizes the impacts of climate change on the United States, now and in the future” (GlobalChange.gov).

Chapter 2, “Our Changing Climate,” reviews the anthropogenic causes of global climate change, as well as the wide-ranging climactic and systemic changes that have been observed from long-term monitoring and projected from experiments and empirical modeling.





The National Climate Assessment & The Global Change Research Program

The National Climate Assessment is a scientific report synthesized by the US Global Change Research Program.  The Program, established by Presidential Initiative in 1989 and mandated by Congress in 1990, is made up of 13 federal agencies that “conduct or use research on global change and its impacts on society." The purpose of the Program is “to assist the Nation and the world to understand, assess, predict and respond to human-induced and natural processes of global change” (GlobalChange.gov).

Chapter Writing Process

Chapter 2: A Changing Climate was ‘based on the collective effort of 32 authors, 3 review editors, and 18 contributing authors.’ The author team was assembled from ‘leading experts in climate science trends and projections, detection and attribution, temperature and precipitation change, severe weather and extreme events, sea level rise and ocean processes, mitigation, and risk analysis.’

Tips for Chapter Review

Each chapter is broken down into sections by “Key Messages.” Some chapters may also begin with an executive summary or introduction before delving into the details of each Key Message. Each one- to two-sentence Key Message is followed by a short section that contains a summary of the literature and data that support and expand upon the message.

At the end of each chapter is a final “Traceable Accounts” section, which gives a brief overview of how the chapter was developed. This section then goes on to address the following for each Key Message: (1) describes where evidence for that key message was drawn from, including major projects and particularly critical studies, (2) acknowledges major uncertainties in conclusions and related data, and (3) classifies scientists’ confidence in conclusions.




Chapter 2 Key Messages

  • Global Climate is Changing Rapidly as a Result of Human Activities
  • Future Warming Depends on Human Emissions and Earth’s Response
  • The Oceans are Changing
  • Global Sea Level Rise
  • Observed and Projected Temperature Change
  • Observed and Projected Precipitation Change
  • Arctic Changes
  • Atmospheric Circulation Patterns are Changing
  • Ocean Circulation, Regional Sea Level Rise and Coastal Flooding
  • Potential Surprises in Future Climate


Key Message 2: Future Warming Depends on Human Emissions and Earth's Response

‘Earth’s climate will continue to change over this century and beyond. Past mid-century, how much climate changes will depend primarily on global emissions of greenhouse gases and on the response of Earth’s climate system to human-induced warming. With significant reductions in emissions, global temperature increase could be limited to 3.6°F (2°C) or less compared to preindustrial temperatures. Without significant reductions, annual average global temperatures could increase by 9°F (5°C) or more by the end of this century compared to preindustrial.’

Cited data and figures include:

  • Feedbacks within the land-ocean-atmosphere system that impact climate projections and climate sensitivity
  • Projected temperature increases under different emission scenarios (if greenhouse gas concentrations stabilized at current level, if emissions continue over time, if emissions were significantly reduced)
  • Figure of observed and project changes in global average temperature alongside projected emissions of carbon dioxide and other heat-trapping gases from human activities

Data sources include:

  • ‘Extensive evidence documented in the climate science literature’
  • Statements made in previous national and international assessments (including the Third National Climate Assessment and the International Panel on Climate Change 2013)

Uncertainties include:

  • ‘Precise magnitude and nature of changes at global, and particularly regional, scales’
  • Feedbacks, particularly ‘ice-albedo and cloud cover feedbacks’


Key Message 3: The Oceans are Changing

The world’s oceans have absorbed 93% of the excess heat from human-induced warming since the mid-20th century and are currently absorbing more than a quarter of the carbon dioxide emitted to the atmosphere annually from human activities, making the oceans warmer and more acidic. Increasing sea surface temperatures, rising sea levels, and changing patterns of precipitation, winds, nutrients, and ocean circulation are contributing.’

Cited data and figures include:

  • Measured increases in heat content and sea surface temperature
  • Measured increases in CO2 and acidity
  • Measured decreases in oxygen levels in inland seas, estuaries, and nearshore coastal waters
  • Projected increases in average sea surface temperature, decreases in ocean oxygen levels, and increasing in surface ocean acidity

Data sources include:

  • Scientific literature (specific reference to Rhein et al. 2013)
  • Data collected via World Ocean Circulation Experiment, Extended Reconstructed Sea Surface Temperature v4 ERSSTv4, satellites, surface drifters and shifts

Uncertainties include: 

  • Magnitude of ocean warming
    • ‘Disparate measurements’ of ocean temperature over the past century
    • Sparse data on warming trends at depths greater than 2,000m
    • Uncertainties in timing and reasons for decadal and interannual variations in ocean heat content, and contributions that different ocean basins play in overall ocean heat uptake
  • Ocean oxygen content
    • Moderate uncertainty related to ‘regional variability driven by mesoscale eddies and intrinsic climate variability such as ENSO’


Key Message 8: Atmospheric Circulation Patterns are Changing

‘Human-induced change is affecting atmospheric dynamics and contributing to the pole-ward expansion of the tropics and the northward shift in Northern Hemisphere winter storm tracks since 1950. Increases in greenhouse gases and decreases in air pollution have contributed to increases in Atlantic hurricane activity since 1970. In the future, Atlantic and eastern North Pacific hurricane rainfall and intensity are projected to increase, as are the frequency and severity of land-falling “atmospheric rivers” on the West Coast.’

Cited data and figures include: 

  • Observed changes in ocean-atmosphere variability and extreme weather events
  • Projections of changes in storm strength and intensity according to climate model simulations

Data sources include:

  • Scientific literature: A ‘large number of studies using a variety of metrics, observations and reanalysis’ to show expansion of the tropics poleward; Modeling studies and theoretical considerations; Recent downscaling studies that provide more fine-scale information on changes in storm intensity and precipitation
  • International Panel on Climate Change AR5 Assessment; International Panel on Climate Change 2013 Assessment
  • Third National Climate Assessment
  • CMIP-based climate change projection studies

Uncertainties include: 

  • Expansion of the tropics
    • Dynamical mechanisms behind changes in the width of the tropical belt
    • How various climate forcings affect the width of the tropic belt
    • Model limitations resulting from horizontal and vertical resolution of global climate models
    • Contribution of natural decadal and multi-decadal variability on observed expansion of the tropics
  • Tropical Cyclones
    • Lack of supporting detectable anthropogenic signal in historical data, to add further confidence to projections
    • Uncertainty in projected pattern and magnitude of future SST
  • Atmospheric Rivers
    • Modest uncertainty in quantifying expected change at a regional level
    • Uncertainty in model ability to represent atmospheric rivers and their interactions with climate
    • Modest uncertainty in lack of a supporting detectable anthropogenic signal in historical data to add further confidence to projections



Are you a scientist at the University of Washington? Attend the UW Program on Climate Change's workshop for this chapter's review. More details here



Note that all single quotes ‘’ cite from the Chapter Two draft text; material in double quotes “” may come from any of the sources below.

"Chapter 2: Our Changing Climate." In Fourth National Climate Assessment. US Global Change Research Program. Chapter Lead: Katharine Hayhoe, Texas Tech University.

"About USGCRP." United States Global Change Research Program. Retrieved from: www.globalchange.gov/about









Contributor: M.S. Student, Aquatic and Fishery Sciences