NCA 4 Draft Report Highlights

Extended draft report highlights

Below are selected highlights and excerpts from the Final Clearance Draft of the U.S. Climate Science Special Report, as posted by the New York Times on August 7, 2017. NOAA led this science assessment, and the report is expected to be published on November 3, 2017, as Volume 1 of the 4th U.S. National Climate Assessment by the U.S. Global Change Research Program.


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, p.160] 

“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 much faster during this time period, than at anytime in the past 1,700 years or more, the time period for which the global distribution of surface temperatures can be reconstructed (high confidence).”

     [Executive Summary, Global and U.S. Temperatures Continue to Rise, page 13.] 

     [See also Figure ES.2 on natural vs. anthropogenic tmperature change, page 14.]

“Since NCA3, stronger evidence has emerged for continuing, rapid, human-caused warming of the global atmosphere and ocean. This reports 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 extant 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 405] 


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

“Human activities are now the dominant cause of the observed trends in climate.”

     [Executive Summary, page 33] 

“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, 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 160] 

“Detectable anthropogenic warming since 1901 has occurred over the western and northern regions of the contiguous United States according to observations and CMIP5 models, 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, p.166] 

“...a number of extreme temperature events (warm anomalies) in the United States have been partly attributed to anthropogenic influence on climate.”
[Section 3.4 Extreme event attribution, page 170] 

“Both anthropogenic climate change and the legacy of land use/management have an influence on U.S. wildfires and are subtly an 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 base on existing studies.” For more detail on western wildfires, see Section 8.3 Wildfires, pages 347-348. 

     [Section 8.3 Wildfires, page 349] 

“For 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 (for example, increased water vapor in a warmer atmosphere).” 

     [Chapter 3, Detection and Attribution of Climate Change, p.166] 

“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 405] 

“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 on surface soil moisture deficits due to increased evapotranspiration caused by higher temperatures.” [Chapter 8, Droughts, Floods, and Wildfires, page 336]. “Agricultural drought describes conditions of soil moisture deficit.” [Section 8.1, Drought, page 337]. “In particular, agricultural drought is of concern to producers of food, while hydrological drought is of concern to water system managers.”

     [Section 8.1, Drought, page 337]. 

“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°F (0.7°C) per century globally since 1900 to 2016. 

     [Chapter 13, Ocean Acidification and Other Ocean Changes, page 540] 

“Human-caused climate change has made a substantial contribution to global mean sea level rise since 1900, with about 3 of those inches (about 7 cm) occurring since 1993. Human-caused climate change has made a substantial contribution to GMSL rise since 1900, contributing to a rate of rise that is greater than during any preceding 7 century in at least 2,800 years.” 

     [Chapter 12, Sea Level Rise, page 493] 

“Coastal flooding during extreme high-water events has become deeper due to local RSL 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, as shown in Figure 12.5a. Locations experiencing such trends changes (based on 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 with several (more than 25) East and Gulf Coast cities...” 

     [Section 12.6, Extreme Water Levels, page 506] 

“...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 Coast over the last 65 years with flooding further compounded by RSL rise.”

     [Section 12.6.4, Extreme Water Levels, page 508] 

“It is virtually certain 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 (very high confidence). Human activities have likely contributed to more than half of the observed arctic surface temperature rise and September sea ice decline since 1979.” 

     [Chapter 3, Detection and attribution of Climate Change, pages 166-167] 

“Human activities have contributed substantially to observed variability in the Atlantic Ocean (medium confidence) and these changes have contributed to the observed upward trend in North Atlantic activity since the 1970s (medium confidence).” [NB: see more detail on hurricanes below] 

     [Chapter 9, Extreme Storms, page 375] 

“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 23] 

“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.” [NB: see more detail on temperatures above]

     [Chapter 3, Detection and Attribution of Climate Change, p.160] 

“Human activities have played a role in the observed expansion of the tropics (by 70 to 200 miles since 1979), although confidence is presently low regarding the magnitude of the human contribution relative to natural variability.”

     [Chapter 3, Detection and Attribution of Climate Change, p.166] 

“The worlds oceans are currently absorbing more than a quarter of the CO2 emitted to the atmosphere annually from human activities, making them more acidic.”

     [Executive Summary, page 29] 

“The rate of acidification in unparalleled in at least 66 million years (medium confidence.”

     [Executive Summary, page 29] 

“Increasingly, climate-induced oxygen loss (deoxygenation) associated with ocean warming and reduced ventilation to deep water has become evident locally, regionally, and globally.

     [Ocean Deoxygenation, section 13.4.1, page 549] 

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 missing (for example, upwelling and circulation) of oxygen in the oceans.”

     [Climate Drivers of Ocean Deoxygenation, section 13.4.2, page 549]


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 variability in the Atlantic Ocean (medium confidence) and these changes have contributed to the observed upward trend in North Atlantic activity since the 1970s (medium confidence).” 

     [Chapter 9, Extreme Storms, page 375] 

“In the Atlantic, observed multidecadal variability of the ocean and atmosphere, which TCs are shown to respond to, has been ascribed (Chapter 3: Detection and Attribution) to [various natural factors] and anthropogenic external forcing via greenhouse gases and sulfate aerosols....” 

     [Chapter 9, Extreme Storms, page 377] 

“Despite the level of disagreement about the relatives magnitude of human influences, there is broad agreement in the literature that human factors (greenhouse gases and aerosols) have had a measurable 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. There is no consensus on the relative magnitude of human and natural influences on past changes in hurricane activity.” 

     [Chapter 9, Extreme Storms, page 377] 

“Projected Changes in Extremes...both physics and numerical simulations (in general) indicated an increase in tropical cyclone intensity in a warmer world, and the models generally show an increase in the number of very intense cyclones.”

     [Executive Summary, page 23] 


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

“Relative to 2000, GMSL (Global Mean Sea Level) is very likely to rise...1-4 feet by 2100 (very high confidence in lower bounds, low confidence in upper bounds)...Emerging science regarding Antarctic Ice Sheet stability suggests that, for high emission scenarios, a GMSL rise exceeding 8 feet by 2100 is physically possible, although the probability of such an extreme outcomes cannot be currently assessed.” 

     [Chapter 12, Sea Level Rise, page 493] 

“...the 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 plausible level of 21st century sea level rise. It is also consistent with the high end projections of Antarctic ice sheet melt discussed below.”

     [Section 12.5, Projected Sea Level Rise, page 499] 

“New observations from any 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 high emissions scenario (RCP8.5), up from 6.6 feet in NCA3.

     [Executive Summary, page 37]


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, RSL 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 493] 

Acidification is regionally greater than the global average along U.S. coastal systems as a result of upwelling (e.g. in the Pacific Northwest) (high confidence), changes in fresh water inputs (e.g. in the Gulf of Maine) (medium confidence), and nutrient input (e.g. in urbanized estuaries) (high confidence). 

     [Executive Summary, page 29] 

“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.”

     [Executive Summary, page 37]


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 additional warming (high confidence). The overall magnitude of the permafrost-carbon feedback is uncertain (Ch. 2); however it is clear that these emissions have the potential to complicate the ability to meet policy goals for the reduction of greenhouse gas concentrations (Ch. 11)

     [Executive Summary, page 30] 

“Recent evidence indicates that permafrost thaw is occurring faster than expected.”

     [Chapter 2, Physical Drivers of Climate Change, page 119]


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 is conducting an unprecedented experiment with the Earth’s climate system through emissions from large-scale fossil-fuel combustion, widespread deforestation, and other changes to the atmosphere and landscape.”

     [Executive Summary, page 35] 

“The present day emissions rate of nearly 10 Gtc per year suggests that there is no climate analog for this century in at least the last 50 million years (medium confidence)."

     [Executive Summary, page 33] 

“...there are still elements of the Earth System that models do not capture well. For this reason, there is significant potential for humankinds’ planetary experiment to results in unanticipated surprises – and the further and faster the Earth’s climate system is changed, the greater the risk of such surprises.” 

     [Executive Summary, page 35] 

“...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.”

     [Executive Summary 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 below 3.6 ̊F (2 ̊C) or lower relative to preindustrial levels requires significant reductions in net global CO2 emissions relative to present-day values before 2040 and likely requires net emissions to become zero or possibly negative later in the century.” 

     [Executive Summary, page 34] 

“Assuming global emissions follow the range between RCP8.5 and RCP4.5 scenarios, emissions could continue for approximately two decades before this cumulative carbon threshold is exceeded.”

     [Executive Summary, page 34] 

Says “first round of Nationally Determined Contributions associated with the Paris Agreement will provide some likelihood of meeting the long-term temperature goal of limited global warming to “well below” 3.6 ̊F (2 ̊C) above preindustrial levels; the likelihood depends strongly on the magnitude of global emissions reductions after 2030. (High confidence). 

     [Executive Summary, page 34] 

“Without major reductions in these emissions, the increase in annual average global temperature relative to pre-industrial times could reach 9 ̊ F (5 ̊ C) or more by the end of this century.”

     [Executive Summary, page 15]