IRISYDHT: The Fourth National Climate Assessment Report

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(IRISYDHT = I Read It So You Don’t Have To)

Human-forced climate change is a delicate topic in many policymaking circles in Texas. I half-jokingly refer to the state of the discussion in the state as an unspoken “Don’t-Ask-Don’t-Tell” policy.” Some folks clearly adhere to the science, some folks clearly don’t, and others have a “wide stance” to navigate the politics.

Regardless of your stance, it’s important to know what the climate scientists—not the policy advocates, not the scientist advocates—are finding on the climate situation. Note that I said “climate scientists.” It’s critical to listen to the specialists in the field, not some non-specialist yahoo that prattles on and on about what is or isn’t happening. It’s also important to listen to scientists (or, even better, a group of scientists) who do not advocate for policy change: in other words, scientists acting like scientists.

Furthermore, it’s key to listen to someone who qualifies their conclusions. I like to say that the best way to detect when a scientist is lying is when they say they are 100 percent positive about something. As my Ph.D. advisor liked to say: “You can never prove a hypothesis; you can only disprove it.” Science, especially climate science, is uncertain. This is not a knock on science—it’s simply the reality of studying the real world.

This fall, the U.S. Global Change Research Program released its Fourth National Climate Assessment Report. The report–put together by a group of scientists–summarizes climate science with a focus on the United States. I read the report (and write about it here) because climate change almost certainly impacts water resource in Texas. Are we seeing climate change in Texas? What might the future hold for Texas? How certain is that science?

The report is well written (although whoever acronymized Tropical Cyclone into TC needs to be kicked in the shins). It’s also well referenced, and it appropriately qualifies (assigns certainty to) each of its conclusions. The report also does a great job of explaining the scientific support–rigourous or not–for each conclusion.

Selfishly, I wish the report would have shown potential impacts to run-off, which is really where the grease hits the griddle on water supplies for both groundwater and surface water in Texas. I also wish they would employ a geographer so they wouldn’t include unprojected maps in the report (a freshman mistake and a major pet peeve of mine).

Below, I’ve pulled out the bits that I found interesting. The bits are, for the most part, verbatim, so you can search on snippets to find them in context in the report to read more. Be forewarned: There are a lot of bits here that I culled from this 470-page report. In a hopeless attempt to increase readibility, I’ve broken the bullets into groups of five.

Stay tuned for subsequent posts where I plan to discuss what the report means for water in Texas.

  • “Global annually averaged surface air temperature has increased by about 1.8°F (1.0°C) over the last 115 years (1901–2016).”
  • “…extremely likely that human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century…”
  • “…global average sea level has risen by about 7–8 inches since 1900, with almost half (about 3 inches) of that rise occurring since 1993…”
  • “Global average sea levels are expected to continue to rise—by at least several inches in the next 15 years and by 1–4 feet by 2100…”
  • “Heavy rainfall is increasing in intensity and frequency across the United States and globally and is expected to continue to increase.”

 

  • “…over the next few decades (2021–2050), annual average temperatures are expected to rise by about 2.5°F for the United States, relative to the recent past (average from 1976–2005)…”
  • “…chronic, long-duration hydrological drought is increasingly possible before the end of this century…”
  • “…the cost of extreme events for the United States has exceeded $1.1 trillion…”
  • “Sixteen of the warmest years on record for the globe occurred in the last 17 years…”
  • “Human activities are the primary driver of recent global temperature rise”

 

  • “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…”
  • “If greenhouse gas concentrations were stabilized at their current level, existing concentrations would commit the world to at least an additional 1.1°F…”
  • “Annual average temperature over the contiguous United States is projected to rise (very high confidence). Increases of about 2.5°F (1.4°C) are projected for the period 2021–2050 relative to the average from 1976–2005 in all RCP scenarios, implying recent record-setting years may be “common” in the next few decades (high confidence). Much larger rises are projected by late century (2071-2100): 2.8°–7.3°F (1.6°–4.1°C) in a lower scenario (RCP4.5) and 5.8°–11.9°F (3.2°–6.6°C) in a higher scenario (RCP8.5) (high confidence)…”
  • “The urban heat island effect will strengthen in the future as the structure and spatial extent as well as population density of urban areas change and grow (high confidence).”
  • “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.”

 

  • “Record warm daily temperatures are occurring more often”
  • “…by geographical scale and duration, the Dust Bowl era of the 1930s remains the benchmark drought and extreme heat event in the historical record…”
  • “Northern Hemisphere spring snow cover extent, North America maximum snow depth, snow water equivalent in the western United States, and extreme snowfall years in the southern and western United States have all declined, while extreme snowfall years in parts of the northern United States have increased.”
  • “Winter storm tracks have shifted northward since 1950 over the Northern Hemisphere (medium confidence)…”
  • “The frequency and intensity of extreme high temperature events are virtually certain to increase in the future as global temperature increases (high confidence).”

 

  • “Extreme precipitation events will very likely continue to increase in frequency and intensity throughout most of the world (high confidence).”
  • “Cold waves are predicted to become less intense while heat waves will become more intense. The number of days below freezing is projected to decline while the number above 90°F will rise. (Very high confidence)”
  • “The frequency and intensity of heavy precipitation events in the United States are projected to continue to increase over the 21st century (high confidence).”
  • “…parts of the southwestern United States are projected to receive less precipitation in the winter and spring (medium confidence)…”
  • “Projections indicate large declines in snowpack in the western United States and shifts to more precipitation falling as rain than snow in the cold season in many parts of the central and eastern United States (high confidence).”

 

  • “Substantial reductions in western U.S. winter and spring snowpack are projected as the climate warms. Earlier spring melt and reduced snow water equivalent have been formally attributed to human-induced warming (high confidence) and will very likely be exacerbated as the climate continues to warm (very high confidence). Under higher scenarios, and assuming no change to current water resources management, chronic, long-duration hydrological drought is increasingly possible by the end of this century (very high confidence)”
  • “Future decreases in surface soil moisture from human activities over most of the United States are likely as the climate warms under the higher scenarios. (Medium confidence)”
  • “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. (High confidence)”
  • “The incidence of large forest fires in the western United States and Alaska has increased since the early 1980s (high confidence) and is projected to further increase in those regions as the climate warms, with profound changes to certain ecosystems (medium confidence)”
  • “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. For Atlantic and eastern North Pacific hurricanes and western North Pacific typhoons, increases are projected in precipitation rates (high confidence) and intensity (medium confidence). The frequency of the most intense of these storms is pro-jected to increase in the Atlantic and western North Pacific (low confidence) and in the eastern North Pacific (medium confidence)”

 

  • “The Arctic is warming at a rate approximately twice as fast as the global average…”
  • “Septembers will be nearly ice-free in the Arctic Ocean sometime between now and the 2040s…”
  • “…the influence of arctic changes on U.S. weather over the coming decades remains an open question with the potential for significant impact.”
  • “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.”
  • “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).”

 

  • “The potential slowing of the Atlantic meridional overturning circulation (AMOC; of which the Gulf Stream is one component)—as a result of increasing ocean heat content and fresh-water-driven buoyancy changes—could have dramatic climate feedbacks as the ocean absorbs less heat and CO2 from the atmosphere.”
  • “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)”
  • “The observed increase in global carbon emissions over the past 15–20 years has been consistent with higher scenarios (e.g., RCP8.5)…”
  • “Global mean atmospheric carbon dioxide (CO2) concentration has now passed 400 ppm, a level that last occurred about 3 million years ago…”
  • “Warming and associated climate effects from CO2 emissions persist for decades to millennia.”

 

  • “In recent decades, land-use and land-cover changes have turned the terrestrial biosphere (soil and plants) into a net “sink” for carbon (drawing down carbon from the atmosphere), and this sink has steadily increased since 1980 (high confidence).”
  • “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.”
  • “…it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.”
  • “Climate, on the other hand, is the statistics of weather…”
  • “…a warmer atmosphere can hold more water vapor…”

 

  • “…the El Niño/La Niña cycle may itself be affected by the human influence on Earth’s climate system.”
  • “Annual averaged precipitation across global land areas exhibits a slight rise (that is not statistically significant because of a lack of data coverage early in the record) over the past century…”
  • “Global atmospheric water vapor should increase by about 6%–7% per °C of warming based on the Clausius–Clapeyron relationship…”
  • “…precipitation increases of approximately 0.55% to 0.72% per °F…”
  • “Earlier studies suggested a climate change pattern of wet areas getting wetter and dry areas getting drier…”

 

  • “While this high/low rainfall behavior appears to be valid over ocean areas, changes over land are more complicated. The wet versus dry pattern in observed precipitation has only been attributed for the zonal mean…”
  • “The detected signal in zonal mean precipitation is largest in the Northern Hemisphere, with decreases in the subtropics and increases at high latitudes.”
  • “…the science of event attribution is rapidly advancing…”
  • “On a global scale, the observational annual-maximum daily precipitation has increased by 8.5% over the last 110 years…”
  • “Extreme precipitation events are increasing in frequency globally over both wet and dry regions…”

 

  • “…extreme precipitation associated with tropical cyclones (TCs) is expected to increase in the future,87 but current trends are not clear…”
  • “…there is low confidence for any significant current trends in river-flooding as-sociated with climate change…”
  • “…but the magnitude and intensity of river flooding is projected to increase in the future.”
  • “…there is evidence for a global increase in severe thunderstorm conditions.”
  • “Winter storm tracks have shifted slightly northward (by about 0.4 degrees latitude) in recent decades over the Northern Hemisphere.”

 

  • “…the locations where tropical cyclones reach their peak intensity have migrated poleward in both the Northern and Southern Hemispheres, in concert with the independently measured expansion of the tropics…”
  • “Tropical cyclone intensities are expected to increase with warming, both on average and at the high end of the scale, as the range of achievable intensities expands, so that the most intense storms will exceed the intensity of any in the historical record…”
  • “Global land-use change is estimated to have released 190 ± 65 GtC (gigatonnes of carbon) through 2015…”
  • “Over the same period, cumulative fossil fuel and industrial emissions are estimated to have been 410 ± 20 GtC, yielding total anthropogenic emissions of 600 ± 70 GtC, of which cumulative land-use change emissions were about 32%…”
  • “…a lengthening growing season, primarily due to the changing climate, and elevated CO2 is expected to further lengthen the growing season in places where the length is water limited…”

 

  • “…global annual averaged temperatures for 1986–2015 are likely much higher, and appear to have risen at a more rapid rate during the last 3 decades, than any similar period possibly over the past 2,000 years or longer…”
  • “…studies of past climates suggest that such global temperatures were likely last observed during the Eemian period—the last interglacial—125,000 years ago; at that time, global temperatures were, at their peak, about 1.8°F–3.6°F (1°C–2°C) warmer than preindustrial temperatures.”
  • “Coincident with these higher temperatures, sea levels during that period were about 16–30 feet (6–9 meters) higher than modern levels…”
  • “Even at equilibrium, internal variability in Earth’s climate system causes limited annual- to decadal-scale variations in regional temperatures and other climate parameters that do not contribute to long-term trends.”
  • Carbonation of finished cement products is a sink of atmospheric CO2…”

 

  • “…offsetting a substantial fraction (0.43) of the industrial-era emissions from cement production…”
  • “Methane is a stronger GHG than CO2 for the same emission mass and has a shorter atmospheric lifetime of about 12 years.”
  • “Contrails and contrail cirrus are additional forms of cirrus cloudiness that interact with solar and thermal radiation to provide a global net positive RF and thus are visible evidence of an anthropogenic contribution to climate change.”
  • “…an increase in low clouds is a negative feedback to RF, while an increase in high clouds is a positive feedback. The potential magnitude of cloud feedbacks is large compared with global RF…”
  • “Cloud feedbacks also influence natural variability within the climate system and may amplify atmospheric circulation patterns and the El Niño–Southern Oscillation.”

 

  • “Basic principles of carbon cycle dynamics in terrestrial ecosystems suggest that increased atmospheric CO2 concentrations can directly enhance plant growth rates and, therefore, increase carbon uptake…”
  • “Detection and attribution at regional scales is generally more challenging than at the global scale for a number of reasons. At the regional scale, the magnitude of natural variability swings are typically larger than for global means…”
  • “When an extreme weather event occurs, the question is often asked: was this event caused by climate change? A generally more appropriate framing for the question is whether climate change has altered the odds of occurrence of an extreme event like the one just experienced.”
  • “To our knowledge, no extreme weather event observed to date has been found to have zero probability of occurrence in a preindustrial climate, according to climate model simulations.”
  • “Hoerling et al. concluded that the 2011 Texas heat wave/meteorological drought was primarily caused by antecedent and concurrent negative rainfall anomalies due mainly to natural variability and the La Niña conditions at the time of the event, but with a relatively small (not detected) warming contribution from anthropogenic forcing.”

 

  • “The anthropogenic contribution nonetheless doubled the chances of reaching a new temperature record in 2011 compared to the 1981–2010 reference period…”
  • “Rupp et al., meanwhile, concluded that extreme heat events in Texas were about 20 times more likely for 2008 La Niña conditions than similar conditions during the 1960s.”
  • “In the illustrative case for the 2011 Texas heat/drought, we conclude that there is medium confidence that anthropogenic forcing contributed to the heat wave, both in terms of a small contribution to the anomaly magnitude and a significant increase in the probability of occurrence of the event.”
  • “If greenhouse gas concentrations were stabilized at their current level, existing concentrations would commit the world to at least an additional 1.1°F (0.6°C) of warming over this century relative to the last few decades (high confidence in continued warming, medium confidence in amount of warming).”
  • “Over the next two decades, global temperature increase is projected to be between 0.5°F and 1.3°F (0.3°–0.7°C) (medium confidence)”

 

  • “Even if existing concentrations could be immediately stabilized, temperature would continue to increase by an estimated 1.1°F (0.6°C) over this century, relative to 1980–1999.”
  • “The resulting range reflects the uncertainty inherent in quantifying human activities (including technological change) and their influence on climate.”
  • “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.”
  • “Recurring patterns of variability in large-scale atmospheric circulation (such as the North Atlantic Oscillation and Northern Annular Mode) and the atmosphere–ocean system (such as El Niño–South-ern Oscillation) cause year-to-year variations in U.S. temperatures and precipitation (high confidence). Changes in the occurrence of these patterns or their properties have contributed to recent U.S. temperature and precipitation trends (medium confidence), although confidence is low regarding the size of the role of human activities in these changes.”
  • “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).”

 

  • “Changes in the occurrence of these patterns or their properties have contributed to recent U.S. temperature and precipitation trends (medium confidence), although confidence is low regarding the size of the role of human activities in these changes.”
  • “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.”
  • “Recurring patterns of variability in large-scale atmospheric circulation (such as the North Atlantic Oscillation and Northern Annular Mode) and the atmosphere–ocean system (such as El Niño–South-ern Oscillation) cause year-to-year variations in U.S. temperatures and precipitation (high confidence). Changes in the occurrence of these patterns or their properties have contributed to recent U.S. temperature and precipitation trends (medium confidence), although confidence is low regarding the size of the role of human activities in these changes.”
  • “On an interannual time scale, coupled atmosphere–ocean phenomena like El Niño–Southern Oscillation (ENSO) have a prominent effect. On longer time scales, U.S. climate anomalies are linked to slow variations of sea surface temperature related to the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO).”
  • “…there are still large uncertainties in our understanding of the impact of human-induced climate change on atmospheric circulation…”

 

  • “…the confidence in any specific projected change in ENSO variability in the 21st century remains low.”
  • “Evidence continues to mount for an expansion of the tropics over the past several decades, with a poleward expansion of the Hadley cell and an associated poleward shift of the sub-tropical dry zones and storm tracks in each hemisphere.”
  • “Recent estimates of the widening of the global tropics for the period 1979–2009 range between 1° and 3° latitude (between about 70 and 200 miles) in each hemisphere, an average trend of between approximately 0.5° and 1.0° per decade.”
  • “Due to human-induced greenhouse gas increases, the Hadley cell is likely to widen in the future, with an accompanying poleward shift in the subtropical dry zones, midlatitude jets, and storm tracks.”
  • “Large uncertainties remain in projected changes in non-zonal to regional circulation components and related changes in precipitation patterns.”

 

  • “Uncertainties in projected changes in midlatitude jets are also related to the projected rate of arctic amplification and variations in the stratospheric polar vortex. Both factors could shift the midlatitude jet equatorward, especially in the North Atlantic region.”
  • “A cooling trend of the tropical Pacific Ocean that resembles La Niña conditions contributed to drying in southwestern North America from 1979 to 2006 and is found to explain most of the decrease in heavy daily precipitation events in the southern United States from 1979 to 2013.”
  • “It is not clear whether observed decadal-scale modulations of ENSO properties, including an increase in ENSO amplitude and an increase in frequency of CP El Niño events are due to internal variability or anthropogenic forcing.”
  • “Climate projections suggest that ENSO will remain a primary mode of natural climate variability in the 21st century…”
  • “Climate models do not agree, however, on projected changes in the intensity or spatial pattern of ENSO.”

 

  • “Model studies suggest an eastward shift of ENSO-induced teleconnection patterns due to greenhouse gas-induced climate change.69, 70, 71, 72 However, the impact of such a shift on ENSO-induced climate anomalies in the United States is not well understood.”
  • “…there is high confidence that, in the 21st century, ENSO will remain a main source of climate variability over the United States on seasonal to interannual timescales.”
  • “There is low confidence for a specific projected change in ENSO variability.”
  • “A negative NAO phase is related to anomalously cold conditions and an enhanced number of cold outbreaks in the eastern United States, while a strong positive phase of the NAO tends to be associated with above-normal temperatures in this region.”
  • “The PDO does not show a long-term trend…”

 

  • “…there is low confidence in projected future changes in the PDO/IPO…”
  • “The North Atlantic Ocean region exhibits coherent multidecadal variability that exerts measurable impacts on regional climate for variables such as U.S. precipitation and Atlantic hurricane activity…”
  • “The internal part of the observed AMV is often referred to as the Atlantic Multidecadal Oscillation (AMO) and is putatively driven by changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC).”
  • “…it is unclear what the statistically derived AMO indices represent, and it is not readily supportable to treat AMO index variability as tacitly representing natural variability…”
  • “Annual average temperature over the contiguous United States has increased by 1.2°F (0.7°C) for the period 1986–2016 relative to 1901–1960 and by 1.8°F (1.0°C) based on a linear regression for the period 1895–2016 (very high confidence). Surface and satellite data are consistent in their depiction of rapid warming since 1979 (high confidence)”

 

  • “Paleo-temperature evidence shows that recent decades are the warmest of the past 1,500 years (medium confidence)”
  • “The frequency of cold waves has decreased since the early 1900s, and the frequency of heat waves has increased since the mid-1960s.”
  • “The Dust Bowl era of the 1930s remains the peak period for extreme heat.”
  • “Annual average temperature over the contiguous United States is projected to rise (very high confidence).”
  • “Increases of about 2.5°F (1.4°C) are projected for the period 2021–2050 relative to 1976–2005 in all RCP scenarios, implying recent record-setting years may be “common” in the next few decades (high confidence).”

 

  • “Much larger rises are projected by late century (2071-2100): 2.8°–7.3°F (1.6°–4.1°C) in a lower scenario (RCP4.5) and 5.8°–11.9°F (3.2°–6.6°C) in the higher scenario (RCP8.5) (high confidence).”
  • “The temperatures of extremely cold days and extremely warm days are both expected to increase.”
  • “The annual average temperature of the contiguous United States has risen since the start of the 20th century. In general, temperature increased until about 1940, decreased until about 1970, and increased rapidly through 2016.”
  • “The largest changes were in the western United States, where average temperature increased by more than 1.5°F (0.8°C) in Alaska, the Northwest, the Southwest, and also in the Northern Great Plains.”
  • “More than 95% of the land surface of the contiguous United States had an increase in annual average temperature…”

 

  • “The decreases in the eastern half of Nation, particularly in the Great Plains, are mainly tied to the unprecedented summer heat of the 1930s Dust Bowl era…”
  • “…there is medium confidence for detectable anthropogenic warming over the western and northern regions of the contiguous United States.”
  • “Annual precipitation has decreased in much of the West, Southwest, and Southeast and increased in most of the Northern and Southern Plains, Midwest, and Northeast. A national average increase of 4% in annual precipitation since 1901 is mostly a result of large increases in the fall season. (Medium confidence)”
  • “Heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901 (high confidence).”
  • “The frequency and intensity of heavy precipitation events are projected to continue to increase over the 21st century (high confidence).”

 

  • “Projections indicate large declines in snowpack in the western United States and shifts to more precipitation falling as rain than snow in the cold season in many parts of the central and eastern United States (high confidence).”
  • “Interannual variability is substantial, as evidenced by large multiyear meteorological and agricultural droughts in the 1930s and 1950s.”
  • “For the contiguous United States, fall exhibits the largest (10%) and most widespread increase, exceeding 15% in much of the Northern Great Plains, South-east, and Northeast. Winter average for the United States has the smallest increase (2%), with drying over most of the western United States as well as parts of the Southeast.”
  • “End-of-season snow water equivalent (SWE)— especially important where water supply is dominated by spring snow melt (for example, in much of the American West)—has declined since 1980 in the western United States, based on analysis of in situ observations, and is associated with springtime warming.”
  • “There is medium confidence that anthropogenic forcing has contributed to global-scale intensification of heavy precipitation over land regions with sufficient data coverage.”

 

  • “…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.”
  • “Although energy constraints can be used to understand global changes in precipitation, projecting regional changes is much more difficult because of uncertainty in projecting changes in the large-scale circulation that plays an important role in the formation of clouds and precip-itation.”
  • “For the contiguous United States (CONUS), future changes in seasonal average precipitation will include a mix of increases, decreases, or little change, depending on location and season…”
  • “High-latitude regions are generally projected to become wetter while the subtropical zone is projected to become drier.”
  • “As the CONUS lies between these two regions, there is significant uncertainty about the sign and magnitude of future anthropogenic changes to seasonal precipitation in much of the region, particularly in the middle latitudes of the Nation.”

 

  • “However, because the physical mechanisms controlling extreme precipitation differ from those controlling seasonal average precipitation, in particular atmospheric water vapor will increase with increasing temperatures, confidence is high that precipitation extremes will increase in frequency and intensity in the future throughout the CONUS.”
  • “…United States, precipitation will decrease in the spring but the changes are only a little larger than natural variations.”
  • “The Third National Climate Assessment projected reductions in annual snowpack of up to 40% in the western United States based on the SRES A2 emissions scenario in the CMIP3 suite of climate model projections.”
  • “Regional model projections of precipitation from landfalling tropical cyclones over the United States, based on downscaling of CMIP5 model climate changes, suggest that the occurrence frequency of post-landfall tropical cyclones over the United States will change little compared to present day during the 21st century, as the reduced frequency of tropical cyclones over the Atlantic domain is mostly offset by a greater landfalling fraction.”
  • “However, when downscaling from CMIP3 model climate changes, projections show a reduced occurrence frequency over U.S. land, indicating uncertainty about future outcomes.”

 

  • “The average tropical cyclone rainfall rates within 500 km (about 311 miles) of the storm center increased by 8% to 17% in the simulations, which was at least as much as expected from the water vapor content increase factor alone.”
  • “Recent droughts and associated heat waves have reached record intensity in some regions of the United States; however, by geographical scale and duration, the Dust Bowl era of the 1930s remains the benchmark drought and extreme heat event in the historical record (very high confidence).”
  • “While by some measures drought has decreased over much of the continental United States in association with long-term increases in precipitation, neither the precipitation increases nor inferred drought decreases have been confidently attributed to anthropogenic forcing.”
  • “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. (High confidence)”
  • “Future decreases in surface (top 10 cm) soil moisture from anthropogenic forcing over most of the United States are likely as the climate warms under higher scenarios. (Medium confidence)”

 

  • “Because potential evapotranspiration increases with temperature, anthropogenic climate change generally results in drier soils and often less runoff in the long term.”
  • “However, even though it happened prior to most of the current global warming, human activities exacerbated the dryness of the soil by the farming practices of the time.”
  • “Climate model simulations suggest that droughts lasting several years to decades occur naturally in the southwestern United States.”
  • “The Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5)7 concluded “there is low confidence in detection and attribution of changes in (meteorological) drought over global land areas since the mid-20th century, owing to observational uncertainties and difficulties in distinguishing decadal-scale variability in drought from long-term trends.””
  • “…there is no detectable change in meteorolog-ical drought at the global scale.”

 

  • “…if a drought occurs, anthropo-genic temperature increases can exacerbate soil moisture deficits…”
  • “The absence of moisture during the 2011 Texas/Oklahoma drought and heat wave was found to be an event whose likelihood was enhanced by the La Niña state of the ocean, but the human interference in the climate system still doubled the chances of reaching such high temperatures.”
  • “This raises the question, as yet unanswered, of whether droughts in the western United States are shifting from precipitation control to temperature control.”
  • “There is some evidence to support a relationship between mild winter and/or warm spring temperatures and drought occurrence…”
  • “Due to its simplicity, the PDSI has been criticized as being overly sensitive to higher temperatures and thus may exaggerate the human contribution to soil dryness.”

 

  • “Seager et al. analyzed climate model output directly, finding that precipitation minus evaporation in the southwestern United States is projected to experience significant decreases in surface water availability, leading to surface runoff decreases in California, Nevada, Texas, and the Colorado River headwaters even in the near term.”
  • “In summary, there has not yet been a formal identification of a human influence on past changes in United States meteorological drought through the analysis of precipitation trends.”
  • “No studies have formally attributed (see Ch. 3: Detection and Attribution) long-term changes in observed flooding of major rivers in the United States to anthropogenic forcing.”
  • “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)”
  • “Both theory and numerical modeling simulations generally indicate an increase in tropical cyclone (TC) intensity in a warmer world, and the models generally show an increase in the number of very intense TCs.”

 

  • “For Atlantic and eastern North Pacific hurricanes and western North Pacific typhoons, increases are projected in precipitation rates (high confidence) and intensity (medium confidence). The frequency of the most intense of these storms is projected to increase in the Atlantic and western North Pacific (low confidence) and in the eastern North Pacific (medium confidence).”
  • “Climate models consistently project environmental changes that would putatively support an increase in the frequency and intensity of severe thunderstorms (a category that combines tornadoes, hail, and winds), especially over regions that are currently prone to these hazards, but confidence in the details of this projected increase is low.”
  • “…it remains likely that global mean tropical cyclone maximum wind speeds and precipitation rates will increase; and it is more likely than not that the global frequency of occurrence of TCs will either decrease or remain essentially the same.”
  • “…the study found no evidence of a connection between the number of major U.S. landfalls from one year to the next and concluded that the 11-year absence of U.S. landfalling major hurricanes was random.”
  • “Other studies have identified systematic interdecadal hurricane track variability that may affect landfalling hurricane and major hurricane frequency.”

 

  • Since the 1970s, the United States has experienced a decrease in the number of days per year on which tornadoes occur, but an increase in the number of tornadoes that form on such days.”
  • “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 (high confidence).”
  • “Recent studies confirm and quantify that surface temperatures are higher in urban areas than in surrounding rural areas for a number of reasons, including the concentrated release of heat from buildings, vehicles, and industry. In the United States, this urban heat island effect results in daytime temperatures 0.9°–7.2°F (0.5°–4.0°C) higher and nighttime temperatures 1.8°– 4.5°F (1.0°–2.5°C) higher in urban areas, with larger temperature differences in humid regions (primarily in the eastern United States) and in cities with larger and denser populations. The urban heat island effect will strengthen in the future as the structure, spatial extent, and population density of urban areas change and grow (high confidence).”
  • “Reducing net emissions of CO2 is necessary to limit near-term climate change and long-term warming.”
  • “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.”

 

  • “The IPCC estimated that 15% to 40% of CO2 emitted un-til 2100 will remain in the atmosphere longer than 1,000 years.”
  • “The persistence of warming is longer than the atmospheric lifetime of CO2 and other GHGs, owing in large part to the thermal inertia of the ocean.”
  • “…heat waves would already be significantly more severe by the 2030s in a non-mitigation scenario compared to a moderate mitigation scenario.”
 [12/12/17: Copyedits]

3 thoughts on “IRISYDHT: The Fourth National Climate Assessment Report

  1. Thanks, Robert! Great work. It demonstrates well the complex interplay of factors and effects that can be cherry-picked and/or misrepresented by deniers and non-scientists, too often to make unfounded inferences and claim things that aren’t true.

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  2. Thanks Robert. I skimmed the ES (executive summary;) of that report and couldn’t get much deeper. Great to have the cliff notes from you (no one under 40 knows what that is).

    You touch on the issue of climate scientists vs scientists advocating policy. It’s a good point, I agree, and appreciate the distinction. Not to split hairs, but because of the politics and interests at play with climate change I see the need for some climate scientists advocating a discussion of the topic and solutions. I suppose they run the risk of being labeled an advocate and perhaps touching the policy realm with talk of solutions. But I don’t see that as a bad thing necessarily in all cases. Perhaps Catherine Hayhoe is the best example–as you know she is a climate scientist and is a great speaker that advocates for broader discussion about climate change. In her talks I believe she discusses limiting carbon and alternative energy as one solution–I think that could be considered a policy–broadly speaking.

    I’m glad there are those in ivory (burnt orange) towers that can ‘just do the science’. But I’m equally glad there are some others like Catherine that do that and talk broadly about the science and about solutions.

    Thanks for your informative, thoughtful, and always entertaining blog. I look forward to your future posts!

    cheers,
    b

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    1. Thanks for the thoughtful comments!

      Scientists can discuss solutions as long as it’s a sober discussion about different possibilities (as is done in this report) or, as you note of Hayhoe, it’s presented as “one possible solution” which acknowledges that there are others. Science in and of itself doesn’t care if we sizzle the Earth like a fried egg; however, a scientist might. And advocates certainly can be scientists, but if they present themselves as scientists first and foremost and advocate for policy, they are doing more harm to science than not. I, for one, don’t want to hear about science from a scientist advocate: It’s too tempting for someone to twist the facts–or omit facts entirely–to support a policy preference.

      imho it’s better for scientists to focus on science and leave the policy advocacy to the professionals. A scientist walking into a heated policy discussion is like going to a knife fight with a pocket protector. It rarely ends well…

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