4th U.S. National Climate Assessment: Notable Findings

Below are selected highlights and excerpts from Volume 1 of the 4th U.S. National Climate Assessment by the U.S. Global Change Research Program, published on November 3, 2017.

Report Highlights

  • The report concludes “it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.” [Executive Summary, p.12] The report further finds that greenhouse gas emissions, deforestation, and other human factors are likely responsible for all of the observed warming since 1951 and that these anthropogenic factors are likely countering and overcoming natural factors that would otherwise be cooling the climate.
  • Deforestation and agriculture have contributed heavily to the warming observed to date.
  • The fingerprints of climate change are now widespread, and climate change is amplifying weather disasters and wildfires.
  • The language describing the impact of climate change on the intensity of hurricanes is now much clearer. Decreases in sulfate aerosols together with increases in GHG emissions are found to be likely contributing to the intensity of hurricanes, however the relative size of these contributions is still an active area of research and debate.
  • The top-end of the “plausible” range for global sea level rise by 2100 has been lifted from 4 feet to 8 feet.
  • The U.S. is particularly vulnerable to rates of sea level rise, ocean acidification, and ocean deoxygenation greater than global rates.
  • A disturbing warming feedback loop has begun in the thawing of Alaskan permafrost.
  • The burning of fossil fuels is having an “unprecedented” impact on the climate and there may be “surprises” with consequences much harsher than currently projected.
  • Limiting global warming to twice the total of warming to date will require a sharp peak and sharp decline in total global carbon pollution before 2040 with pollution from the burning of fossil fuels eventually stopping entirely.

Corresponding Excerpts and Page Citations

Carbon pollution, deforestation, and other human factors are likely responsible for all of the observed warming since 1951. In addition these anthropogenic factors are likely countering and overcoming natural factors that would otherwise be cooling the climate.

  • “The likely range of the human contribution to the global mean temperature increase over the period 1951–2010 is 1.1° to 1.4°F (0.6° to 0.8°C), and the central estimate of the observed warming of 1.2°F (0.65°C) lies within this range (high confidence). This translates to a likely human contribution of 93%– 123% of the observed 1951–2010 change.”
    [Chapter 3, Detection and Attribution of Climate Change, page 114]
  • “Longer-term climate records over past centuries and millennia indicate that average temperatures in recent decades over much of the world have been much higher, and have risen faster during this time period, than at any time in the past 1,700 years or more, the time period for which the global distribution of surface temperatures can be reconstructed. (High confidence)”
    [Chapter 1, Our Globally Changing Climate, page 36; see also Figure 2 in the Executive Summary illustrating the impact of natural vs. anthropogenic contributions to global temperature change, page 14]
  • “Since NCA3, stronger evidence has emerged for continuing, rapid, human-caused warming of the global atmosphere and ocean. This report concludes that “it is extremely likely that human influence has been 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.”
    [Executive Summary, page 12]

Deforestation and agriculture have contributed heavily to the warming to date.

  • “Changes in land use and land cover due to human activities produce physical changes in land surface albedo, latent and sensible heat, and atmospheric aerosol and greenhouse gas concentrations. The combined effects of these changes have recently been estimated to account for 40% ± 16% of the human-caused global radiative forcing from 1850 to present day.”
    [Chapter 10, Changes in Land Cover and Terrestrial Biogeochemistry, page 277]

The fingerprints of global warming are now widespread. And climate change is now amplifying weather disasters and wildfires.

  • “[I]t is extremely likely that human influence has been 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.”
    [Executive Summary, page 12]
  • “The last few years have also seen record-breaking, climate related weather extremes, the three warmest years on record for the globe, and continued decline in arctic sea ice. These trends are expected to continue into the future over climate (multidecadal) timescales.”
    [Executive Summary, page 12]
  • “Since the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) chapter on detection and attribution and the Third National Climate Assessment (NCA3), the science of detection and attribution has advanced, with a major scientific question being the issue of attribution of extreme events.”
    [Chapter 3, Detection and Attribution of Climate Change, page 115]
  • “Detectable anthropogenic warming since 1901 has occurred over the western and northern regions of the contiguous United States according to observations and CMIP5 models (medium confidence), although over the southeastern United States there has been no detectable warming trend since 1901. The combined influence of natural and anthropogenic forcings on temperature extremes have been detected over large subregions of North America.”
    [Chapter 3, Detection and Attribution of Climate Change, page 121]
  • “…a number of extreme temperature events (warm anomalies) in the United States have been partly attributed to anthropogenic influence on climate.”
    [Chapter 3, Detection and Attribution of Climate Change, page 170]
  • “Both anthropogenic climate change and the legacy of land use/management have an influence on U.S. wildfires and are subtly and inextricably intertwined . . . there is medium confidence for a human-caused climate change contribution to increased forest fire activity in Alaska in recent decades . . . and low to medium confidence for a detectable human climate change contribution in the western United States based on existing studies.”
    [Chapter 8, Droughts, Floods, and Wildfires, page 224]
  • “[F]or the continental United States there is high confidence in the detection of extreme precipitation increases, while there is low confidence in attributing the extreme precipitation changes purely to anthropogenic forcing. There is stronger evidence for a human contribution (medium confidence) when taking into account process-based understanding (increased water vapor in a warmer atmosphere), evidence from weather and climate models, and trends in other parts of the world.”
    [Chapter 7, Precipitation Change in the United States, page 214]
  • “Climate change and induced changes in the frequency and magnitude of extreme events (e.g., droughts, floods, and heat waves) have led to large changes in plant community structure with subsequent effects on the biogeochemistry of terrestrial ecosystems.”
    [Chapter 10, Changes in Land Cover and Terrestrial Biogeochemistry, page 227]
  • “The human effect on recent major U.S. droughts is complicated. Little evidence is found for a human influence on observed precipitation deficits, but much evidence is found for a human influence on surface soil moisture deficits due to increased evapotranspiration caused by higher temperatures.” [Chapter 8, Droughts, Floods, and Wildfires, page 231]
  • “‘Agricultural drought’ describes conditions of soil moisture deficit. ‘Hydrological drought’ describes conditions of deficit in runoff . . . In particular, agricultural drought is of concern to producers of food while hydrological drought is of concern to water system managers.”
    [Chapter 8, Droughts, Floods, and Wildfires, page 232]
  • “The world’s oceans have absorbed about 93% of the excess heat caused by greenhouse gas warming since the mid-20th century, making them warmer and altering global and regional climate feedbacks. Ocean heat content has increased at all depths since the 1960s and surface waters have warmed by about 1.3° ± 0.1°F (0.7° ± 0.08°C) per century globally since 1900 to 2016.”
    [Chapter 13, Ocean Acidification and Other Ocean Changes, page 364]
  • “Global mean sea level (GMSL) has risen by about 7–8 inches (about 16–21 cm) since 1900, with about 3 of those inches (about 7 cm) occurring since 1993 (very high confidence). Human-caused climate change has made a substantial contribution to GMSL rise since 1900 (high confidence), contributing to a rate of rise that is greater than during any preceding century in at least 2,800 years (medium confidence).”
    [Chapter 12, Sea Level Rise, page 333]
  • “Coastal flooding during extreme high-water events has become deeper due to local RSL rise and more frequent from a fixed-elevation perspective. Trends in annual frequencies surpassing local emergency preparedness thresholds for minor tidal flooding (i.e., “nuisance” levels of about 30–60 cm [1–2 feet]) that begin to flood infrastructure and trigger coastal flood “advisories” by NOAA’s National Weather Service have increased 5- to 10-fold or more since the 1960s along the U.S. coastline . . . Locations experiencing such trend changes (based upon fits of flood days per year of Sweet and Park 2014) include Atlantic City and Sandy Hook, NJ; Philadelphia, PA; Baltimore and Annapolis, MD; Norfolk, VA; Wilmington, NC; Charleston, SC; Savannah, GA; Mayport and Key West, FL; Port Isabel, TX, La Jolla, CA; and Honolulu, HI. In fact, over the last several decades, minor tidal flood rates have been accelerating within several (more than 25) East and Gulf Coast cities…”
    [Chapter 12, Sea Level Rise, page 347]
  • “…there have been more storms producing concurrent locally extreme storm surge and rainfall (not captured in tide gauge data) along the U.S. East and Gulf Coasts over the last 65 years, with flooding further compounded by local RSL rise.”
    [Chapter 12, Sea Level Rise, page 349]
  • “[I]t is very likely that human activities have contributed to arctic surface temperature warming, sea ice loss since 1979, glacier mass loss, and Northern Hemisphere snow extent decline observed across the Arctic. Key uncertainties remain in the understanding and modeling of arctic climate variability; however, many independent studies indicate that internal variability alone cannot explain the trends or extreme events observed in arctic temperature and sea ice over the satellite era.”
    [Chapter 11, Arctic Changes and their Effects on Alaska and the Rest of the United States, pages 319-320]
  • “Human activities have contributed substantially to observed ocean–atmosphere variability in the Atlantic Ocean (medium confidence), and these changes have contributed to the observed upward trend in North Atlantic hurricane activity since the 1970s (medium confidence).”
    [Chapter 9, Extreme Storms, page 257; see more detail on hurricanes below]
  • “Some storm types such as hurricanes, tornadoes, and winter storms are also exhibiting changes that have been linked to climate change, although the current state of the science does not yet permit detailed understanding.”
    [Executive Summary, page 19]
  • “Earlier spring melt and reduced snow water equivalent have been formally attributed to human-induced warming (high confidence)…”
    [Executive Summary, page 22]
  • “The likely range of the human contribution to the global mean temperature increase over the period 1951–2010 is 1.1° to 1.4°F (0.6° to 0.8°C), and the central estimate of the observed warming of 1.2°F (0.65°C) lies within this range (high confidence). This translates to a likely human contribution of 93%–123% of the observed 1951–2010 change.”
    [Chapter 3, Detection and Attribution of Climate Change, page 114; see more detail on temperatures above]
  • “The tropics have expanded poleward by about 70 to 200 miles in each hemisphere over the period 1979–2009, with an accompanying shift of the subtropical dry zones, midlatitude jets, and storm tracks (medium to high confidence). Human activities have played a role in this change (medium confidence), although confidence is presently low regarding the magnitude of the human contribution relative to natural variability.”
    [Chapter 5, Large-Scale Circulation and Climate Variability, 161]
  • “The world’s oceans are currently absorbing more than a quarter of the CO2 emitted to the atmosphere annually from human activities, making them more acidic (very high confidence), with potential detrimental impacts to marine ecosystems.”
    [Chapter 13, Ocean Acidification and Other Ocean Changes, page 364]
  • “The rate of acidification is unparalleled in at least the past 66 million years (medium confidence).”
    [Chapter 13, Ocean Acidification and Other Ocean Changes, page 364]
  • “Increasingly, climate-induced oxygen loss (deoxygenation) associated with ocean warming and reduced ventilation to deep waters has become evident locally, regionally, and globally.”
    [Chapter 13, Ocean Acidification and Other Ocean Changes, page 375]
  • “Global ocean deoxygenation is a direct effect of warming. Ocean warming reduces the solubility of oxygen (that is, warmer water can hold less oxygen) and changes physical mixing (for example, upwelling and circulation) of oxygen in the oceans.”
    [Chapter 13, Ocean Acidification and Other Ocean Changes, page 375]

The language describing the impact of climate change on the intensity of hurricanes is now much clearer. Decreases in sulfate aerosols together with increases in GHG emissions are found to be likely contributing to the intensity of hurricanes, however the relative size of these contributions is still an active area of research and debate.

  • “Human activities have contributed substantially to observed ocean–atmosphere variability in the Atlantic Ocean (medium confidence), and these changes have contributed to the observed upward trend in North Atlantic hurricane activity since the 1970s (medium confidence).”
    [Chapter 9, Extreme Storms, page 268]
  • “In the Atlantic, observed multidecadal variability of the ocean and atmosphere, which TCs are shown to respond to, has been ascribed (Ch. 3: Detection and Attribution) to [various natural factors] and anthropogenic external forcing via greenhouse gases and sulfate aerosols.”
    [Chapter 9, Extreme Storms, page 259]
  • “Despite the level of disagreement about the relative magnitude of human influences, there is broad agreement that human factors have had an impact on the observed oceanic and atmospheric variability in the North Atlantic, and there is medium confidence that this has contributed to the observed increase in hurricane activity since the 1970s.”
    [Chapter 9, Extreme Storms, page 259]
  • “Both physics and numerical modeling simulations generally indicate an increase in tropical cyclone intensity in a warmer world, and the models generally show an increase in the number of very intense tropical cyclones.”
    [Executive Summary, page 22]

The top-end of the “plausible” range for sea level rise by 2100 has been lifted from 4 feet to 8 feet.

  • “Relative to the year 2000, GMSL is very likely to rise . . . 1.0–4.3 feet (30–130 cm) by 2100 . . . Emerging science regarding Antarctic ice sheet stability suggests that, for higher scenarios, a GMSL rise exceeding 8 feet (2.4 m) by 2100 is physically possible, although the probability of such an extreme outcome cannot currently be assessed.”
    [Chapter 12, Sea Level Rise, page 333]
  • “[T]he U.S. Interagency Sea Level Rise Task Force has revised the GMSL rise scenarios for the United States and now provides six scenarios that can be used for assessment and risk-framing purposes… the highest scenario of 250 cm (8.2 feet) is consistent with several literature estimates of the maximum physically plausible level of 21st century sea level rise…t is It is also consistent with the high end of recent projections of Antarctic ice sheet melt…”
    [Chapter 12, Sea Level Rise, page 342]
  • “New observations from many different sources confirm that ice-sheet loss is accelerating. Combining observations with simultaneous advances in the physical understanding of ice sheets leads to the conclusion that up to 8.5 feet of global sea level rise is possible by 2100 under a higher scenario (RCP8.5), up from 6.6 feet in NCA3.”
    [Executive Summary, page 34; see also Chapter 12]

The U.S. is particularly vulnerable to rates of sea level rise, ocean acidification, and ocean deoxygenation greater than global rates.

  • “For almost all future GMSL rise scenarios, relative sea level rise is likely to be greater than the global average in the U.S. Northeast and the western Gulf of Mexico…Almost all U.S. coastlines experience more than global mean sea level rise in response to Antarctic ice loss, and thus would be particularly affected under extreme GMSL rise scenarios involving substantial Antarctic mass loss (high confidence).”
    [Chapter 12, Sea Level Rise, page 333]
  • “Acidification is regionally increasing along U.S. coastal systems as a result of upwelling (for example, in the Pacific Northwest) (high confidence), changes in freshwater inputs (for example, in the Gulf of Maine) (medium confidence), and nutrient input (for example, in agricultural watersheds and urbanized estuaries) (high confidence).”
    [Chapter 13, Ocean Acidification and Other Ocean Changes, page 364]
  • “Both oxygen loss and acidification may be magnified in some U.S. coastal waters relative to the global average, raising the risk of serious ecological and economic consequences.”
    [Chapter 1, Our Globally Changing Climate, page 57]

 

A warming feedback loop has begun in the thawing of Alaskan permafrost

  • “Rising Alaskan permafrost temperatures are causing permafrost to thaw and become more discontinuous; this process releases additional carbon dioxide and methane, resulting in an amplifying feedback and additional warming (high confidence). The overall magnitude of the permafrost–carbon feedback is uncertain; however, it is clear that these emissions have the potential to compromise the ability to limit global temperature increases.”
    [Chapter 11, Arctic Changes and their Effects on Alaska and the Rest of the United States page 30]
  • “Recent evidence indicates that permafrost thaw is occurring faster than expected.”
    [Chapter 2, Physical Drivers of Climate Change, page 95]

The burning of fossil fuels is having an “unprecedented” impact on the climate and there may be “surprises” with consequences much harsher than currently projected.

  • “Humanity’s effect on the Earth system, through the large-scale combustion of fossil fuels and widespread deforestation and the resulting release of carbon dioxide (CO2) into the atmosphere, as well as through emissions of other greenhouse gases and radiatively active substances from human activities, is unprecedented.”
    [Executive Summary, page 32]
  • “The present-day emissions rate of nearly 10 GtC per year suggests that there is no climate analog for this century any time in at least the last 50 million years (medium confidence).”
    [Chapter 4, Climate Models, Scenarios, and Projections, page 151]
  • “There is significant potential for humanity’s effect on the planet to result in unanticipated surprises and a broad consensus that the further and faster the Earth system is pushed towards warming, the greater the risk of such surprises.”
    [Executive Summary, page 32]
  • “While climate models incorporate important climate processes that can be well quantified, they do not include all of the processes that can contribute to feedbacks, compound extreme events, and abrupt and/or irreversible changes. For this reason, future changes outside the range projected by climate models cannot be ruled out (very high confidence).”
    [Executive Summary, page 33]
  • “…the systematic tendency of climate models to underestimate temperature change during warm paleoclimates suggests that climate models are more likely to underestimate than to overestimate the amount of long-term future change (medium confidence).”
    [Chapter 15, Potential Surprises: Compound Extremes and Tipping Points, page 36]

Limiting global warming to twice the total of warming to date will require a sharp peak and sharp decline in total global carbon pollution before 2040 with pollution from the burning of fossil fuels eventually stopping entirely.

  • “Stabilizing global mean temperature to less than 3.6°F (2°C) above preindustrial levels requires substantial reductions in net global CO2 emissions prior to 2040 relative to present-day values and likely requires net emissions to become zero or possibly negative later in the century.”
    [Chapter 14, Perspectives on Climate Change Mitigation, page 393]
  • “Assuming global emissions are equal to or greater than those consistent with the RCP4.5 scenario, this cumulative carbon threshold would be exceeded in approximately two decades. (High confidence)”
    [Chapter 14, Perspectives on Climate Change Mitigation, page 393]
  • “Achieving global greenhouse gas emissions reductions before 2030 consistent with targets and actions announced by governments in the lead up to the 2015 Paris climate conference would hold open the possibility of meeting the long-term temperature goal of limiting global warming to 3.6°F (2°C) above preindustrial levels, whereas there would be virtually no chance if net global emissions followed a pathway well above those implied by country announcements. Actions in the announcements are, by themselves, insufficient to meet a 3.6°F (2°C) goal; the likelihood of achieving that depends strongly on the magnitude of global emissions reductions after 2030. (High confidence).”
    [Executive Summary, page 32]
  • “Without major reductions in these emissions, the increase in annual average global temperatures relative to preindustrial times could reach 9°F (5°C) or more by the end of this century (high confidence).”
    [Executive Summary, page 35]
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