Kevin Trenberth’s Climate Attribution Studies versus Useful Science - Part 1 Hurricane Sand

Useful climate science helps humanity adapt to natural weather patterns and plan for future extremes. Minimizing risk would be a wise course of action, but too often humans have ignored nature’s indicators of high flood risk and continue to build on flood plains that inevitably place them in harms way. By the 1950s most private companies in America got out of the flood insurance business due to heavy losses from natural flooding. In 1968 the US government unwisely decided to subsidize flood insurance and created the National Flood Insurance Program. One unintended consequence was subsidies encouraged people to continue building in flood plains. As a consequence, by 2014 the program was $24 billion in debt. Recent legislation has attempted to raise flood insurance rates to better reflect the real risks. But raising insurance rates to reflect real risks could force many homeowners into foreclosure and thus any meaningful solution creates a political nightmare. Whatever the political solution, accurate risk assessments require hydrologists and climate scientists to determine the frequency of major flood producing storms over hundreds of years. In Attribution of Extreme Climate Events (henceforth Trenberth 2015) Trenberth suggests extreme storms are more frequent due to global warming. But from a perspective of several centuries, we know flood risks due to hurricanes were greater during the cooler climate of the Little Ice Age (LIA). So how valid and useful is the science of Trenberth 2015?

Textbooks published years before the landfall of Hurricanes Katrina or Sandy prophetically warned, “New Orleans lies below sea level. Should hurricane-driven floods top or break the protecting levees, the city would be inundated with seawater.” And “Large parts of Long Island, New York with its very large population, would be underwater if a major hurricane passed over its western end.” Simply knowing that there was a greater risk of hurricane-induced flooding during the LIA is not really useful for those cities. They already know they are naturally in danger.  Those cities require early warning systems to allow safe evacuation. Accurate early warning requires useful science that can predict the effects of atmospheric circulation and determine storm tracks, storm duration and storm intensity. Oddly Trenberth 2015 argued we should separate analyses of those most useful dynamics and focus on thermodynamics (temperature) because CO2 forced circulation models do a very poor job of simulating those critical dynamic changes. Trenberth 2015 wants to focus on the effect of temperature anomalies in isolation to provide “a better basis for communication of climate change to the public.” But examining temperature anomalies separate from atmospheric circulation changes is dubious science at best and blaming global warming does nothing to improve early storm warnings or accurately assess the frequency of extreme events.

Centennial and Millennial Hurricane Storm Surge

Trenberth 2015 suggested that for Hurricane Sandy, “the subways and tunnels may not have flooded without warming-induced increase in sea level and storm intensity and size, putting a potential price tag of human climate change in this storm in the tens of billions of dollars.” [approaching 50% of the damage].But changes in sea level had little, if any, impact on Sandy’s flooding. His statement may be useful for politicking climate change but does nothing to improve early warning systems. The more useful question to have asked is why was Sandy’s storm surge double that of recent hurricanes, hurricanes that were far more intense but with similar sea levels?

Examining the graphic on storm surge (below) posted by one of Trenberth’s colleagues at the National Center for Atmospheric Research, we clearly see how extreme high water events since 1900 are broken down into contributions from storm surge, high tides and a century of sea level rise. The diagonal orange & white areas represent sea level rise since 1900.  Sea level at Battery Park, NY has risen 11.2 inches over the past 100 years. Half of that rise happened naturally by 1950 - before CO2 had reached significant concentrations - and that natural sea level rise has most likely continued into the present to some degree. Furthermore due to glacial isostatic adjustments, 3 to 4 inches of that relative sea level rise is due to land subsidence on the eastern seaboard. Thus any theoretical contribution from human warming to current sea level is most likely less than 3 inches, and less than 3% of Sandy’s high-water levels. Even if we incorrectly assumed that CO2 caused the entire 1-foot rise in sea level, if we remove that sea level increase Sandy would have still flooded New York’s subways. By blaming global warming, Trenberth 2015 provided nothing useful that would have predicted Sandy’s flooding.

In contrast to Trenberth’s global warming crusade, paleo-climate studies of storm-washed sediments in New York City’s back-barrier marshes show high storm surge was more common when the climate was cooler and sea level was lower. As seen below in Figure 5, coastal flooding similar to Sandy’s happened in 1788, 1821 and 1893. The conclusions from sediment analyses are further supported by historical documentation. The 1893 storm surge was reported to have destroyed Hog Island while driving large boats 100s of feet inland.

Because hurricane caused flooding was more prevalent during the Little Ice Age when Atlantic temperatures averaged 1 to 2 degrees F colder than today researchers concluded, “The frequent occurrence of major hurricanes in the western Long Island record suggests that other climate phenomena, such as atmospheric circulation, may have been favorable for intense hurricane development despite lower sea surface temperatures.” In contrast Trenberth 2015 incorrectly argued analyzing the causes of atmospheric circulation anomalies is not as “fruitful” as analyzing temperatures.

Similarly Liu and Fearn 2000 investigated storm-washed sediments in northern Florida, concluding the region was afflicted with millennial periods of hyperactivity for extreme hurricanes that alternated with a thousand years of quiescent activity. They reported that “no catastrophic hurricane of category 4 or 5 intensity has made landfall in the Western Lake [northern Florida] area during the last 130 year documentary record” but “If future climatic changes, whether or not related to the anticipated greenhouse warming, lead to a return of a “hyperactive” hurricane regime characteristic of the first millennium A.D., then the northeastern Gulf Coast is expected to experience a dramatic increase in the frequency of strikes by catastrophic hurricanes.” Globally other paleo-climate studies found the period of greatest hurricane activity for Australia and the eastern USA both occurred during Little Ice Age times between 1400 and 1800 AD. And in Southeast Asia researchers determined “the two periods of most frequent typhoon strikes in Guangdong (AD 1660–1680, 1850–1880) coincided with two of the coldest and driest periods in northern and central China during the Little Ice Age.”  

Trenberth 2015 wants to re-direct research questions and ask, “Given an extreme storm, how was it influenced by anomalous SSTs?” or “ Was the storm surge worse because of high sea levels?” Based on long-term studies the answer is extreme storms and high storm surge happened more frequently with cooler sea surface temperatures and long before rising CO2. Given that NYC experienced 3 extreme high water levels associated with hurricanes between 1788 and 1893, but only one (Sandy) since then, we can reasonably argue that climate change, whether human-induced or natural, has reduced the threat of high storm surge.

Storm Tracks and Storm Surge

Early warnings and evacuation plans critically hinge on projected storm surge, which primarily depend on the projected storm track.  Perusing hurricane storm tracks since 1850 (illustration below) reveals it was Sandy’s unusual perpendicular approach to the coast that enhanced storm surge. Consider the more intense Hurricane of 1938, which made landfall on Long Island slightly north of New York City as a more intense category 3 hurricane, implying sustained wind speeds between 111 and 130 miles per hour. In contrast Hurricane Sandy made landfall in New Jersey slightly south of New York City as an extra-tropical storm implying winds speeds less than 74 mph. Yet the more powerful hurricane of 1938 only generated maximum water levels at Battery Park, NY of 8.8 feet, and does not make New York City’s top ten high water levels over the past 100 years. It was the difference in storm tracks that determined Sandy’s higher storm surge and higher costs.

Hurricanes produce the highest winds to the right of the hurricane’s direction of travel. Storms travelling parallel to the coastline don’t aim the strongest winds at the coast. When Sandy took a 90-degree turn and travelled perpendicular to the coast, she aimed her most powerful winds at New York City for a more extended period of time as she approached. Due to Sandy’s more eastward position when she started her approach, the fetch was also greater and generated much bigger swells.

The degree of storm surge also depends upon how quickly a storm moves up the coast. Sandy was a hybrid storm that had merged with a cold-core extra-tropical storm typical of winter Nor’easters. Unlike hurricanes that are powered by latent heat from warm sea surfaces, extra-tropical winter storms along the eastern seaboard are primarily powered by the pressure gradient produced by the contrast between the cold continent and warm Gulf Stream. While Trenberth only draws your attention to anomalously warm sea surface temperatures, the east coast was experiencing record cold temperatures that increased the pressure gradient. Forecasters were issuing both blizzard and hurricane watches. Furthermore extra-tropical storms are 3 to 4 times wider than hurricanes, and merging with Sandy produced the hybrid hurricane’s immense size. Extra-tropical storms and their hybrids move much more slowly up the coast than a hurricane, thus the duration of Sandy’s winds generated a much greater storm surge. As seen in Figure 5 above, extra-tropical winter storms (light gray bars) have produced the greatest abundance extreme storm surge. Apportioning partial causation of Sandy’s destruction on global warming and ignoring all else only obscures the critical dynamics required to make early warning predictions based on storm intensity.

Atmospheric Blocking

Due to the frequency of failed forecasts, the public often dismisses media hype about the dangers of an approaching storm, preferring to stay and take their chances rather than needlessly evacuate. Carelessly blaming global warming only adds to the dubious hype and mistrust of useful science. Fortunately the European ECMWF weather models accurately forecasted Sandy’s storm track 8 to 9 days in advance due to a better understanding of atmospheric blocking (in this case the high pressure south of Greenland) and the effects of the jet stream. In contrast, the American National Weather Service’s GFS models initially forecast Sandy to harmlessly head out to the mid Atlantic. Due to such poor forecasting skills, Congress appropriated funds so the NWS could adopt a more accurate weather model. Why did models differ so greatly in forecasting Sandy’s storm track? All the models had access to the same sea surface temperature data, so Trenberth’s temperature anomalies were never a critical factor that could explain model differences.

In fact Trenberth 2015 cited Magnusson 2014 (a paper Trenberth helped craft) in which a ECMWF modeling experiment compared the most recent 20-year average sea surface temperatures with a swath of the Atlantic’s anomalously high temperatures during Sandy’s northward trek. Although that experiment suggested anomalous temperatures could have possibly increased storm intensity slightly, forecasting intensity is still fraught with problems due to the complex contributions from many other variables. More importantly the ECMWF experiment found changes in sea surface temperature had little effect on Sandy’s storm track. Model runs with failed forecasts underestimated the strength of the subtropical high-pressure systems east of the storm track that had kept Sandy from harmlessly veering into the Atlantic. In contrast to Trenberth’s 2015 lament that atmospheric circulation patterns are not robustly simulated by CO2-driven climate models, predicting storm tracks and blocking are the most critical factors for providing early warnings.

Figure 1   Trend in Blocking Days from  Hakkinen 2011

If researchers are interested in a link between Sandy’s storm track and climate change, then a better question to ask would be  ‘have Greenland blocking events been affected by rising CO2 and climate change?” As illustrated in Figure 1 above from the 2011 paper Atmospheric Blocking and Atlantic Multidecadal Ocean Variability, the answer would be there has been no trend in Greenland blocking days (estimates in black and dark blue). Thus a CO2 global warming effect is again unlikely. In contrast, Greenland blocking and hurricane activity are both significantly associated with natural oscillations like the Atlantic Mulitdecadal Oscillation (AMO). The dashed red line represents the AMO and the solid red line represents the detrended AMO.

Klotzbach 2015 has shown that hurricane activity in the Atlantic is highly correlated with the AMO and seemingly independent of climate change. Three peaks of the AMO coincide with 3 peaks of hurricane activity centered on the 1880s, 1950s, and 2005. The oscillation of Atlantic hurricane activity is also illustrated in the Accumulated Cyclone Energy (ACE) index shown below. The AMO appears to be transitioning towards its cool phase now coinciding with a period of below average hurricane activity in the Atlantic since 2013.

Storm Intensity

According to Trenberth’s 2007 article Warmer Ocean’s, Stronger Hurricanes, a one-degree increase in sea surface temperature can increase the winds of a hurricane by one category and he argued global warming will produce more intense category 4 and 5 hurricanes. But Sandy only briefly reached category 3 status as she approached Cuba. Sandy quickly lost intensity after passing over Cuba, devolving from a category 3 hurricane to a mere extra-tropical storm before strengthening again to a weak category 1 hurricane. Clearly the ocean was not warm enough to produce a higher intensity storm that Trenberth and global warming predicted. Or perhaps the dynamic factors that Trenberth downplays had a more powerful part in limiting Sandy’s intensity.

Days before making landfall, due to Sandy’s more westerly storm track, Sandy interacted with an atmospheric trough and its cold Arctic air mass that had dipped down over the eastern USA. The warm-core hurricane named Sandy, eventually merged with a cold-core extra-tropical storm generated by the jet stream. While tropical hurricane intensity is primarily driven by latent heat from warm sea surface temperatures, an extra-tropical storm is primarily driven by baroclinic processes (differences in the pressure gradient) such as the gradient due to the contrast between the warm Gulf Stream and cold continental air mass. As Magnussen 2014 noted, when tropical cyclones and mid-latitude troughs interact to form a “hybrid storm”, it has been found that cyclones are more likely to intensify than weaken. Thus it can be reasonably argued that it was abnormally cold continental temperatures that intensified Sandy.

During a hurricane’s typical cold-induced extra-tropical transition, a hurricane’s size greatly increases as observed in the extremely large radius of Sandy. As reported by Galarneaux 2013, during the transition winds increased by 20% and Sandy’s central pressure dropped to its lowest point of 940 hPa despite travelling over cooler waters. During her “second trough interaction on 29 October, Sandy turned northwestward and intensified as cold continental air encircled the warm core vortex.”  

Everyone agrees that hurricanes require warm waters to form and indeed warmer temperatures can intensify a hurricane. As seen in Figure 10 below, Sandy’s storm track crossed the Gulf Stream (the reddish bands) before making landfall. As she crossed the Gulf Stream, she briefly intensified to a Category 2  hurricane before devolving again to an extra-tropical storm when she crossed cooler coastal waters (in blue and purple). But here again Trenberth’s attempt to separate the dynamics of atmospheric circulation from a thermodynamic impact of higher temperatures would be misleading. It was blocking that forced Sandy to cross over the naturally warm waters of the Gulf Stream. Sandy’s brief increase in intensity was ultimately the result of atmospheric circulation not global warming. Otherwise she would have passed harmlessly out to sea.

The attempts by Trenberth 2015 to suggest global warming has worsened disasters like Hurricane Sandy or the Colorado flooding (discussed in part 2) simply fails to provide any useful science. Trenberth 2015 did not accurately assesses risks or improve early warning systems. It simply reduced climate science to “ambulance chasing” in order to scare up support for his climate change politicking. Previously Trenberth has argued that extreme events such as recent droughts and heat waves worsened due to CO2 warming and despite the fact that climate experts found those events to be within the bounds of natural variability (discussed here). To communicate his brand of climate change, Trenberth attacked those scientists on blogs as irresponsible. And here again Trenberth has hyped global warming links to hurricane destruction in contrast to the opinions of many hurricane experts.

In keeping with the long-term framework required by climate science, hurricane experts like Chris Landsea, the late Bill Gray and Jim O'Brien have consistently reported there are no links between global warming and hurricanes. All the evidence such as the recent lull in Atlantic hurricane activity supports their claims. But despite not being a hurricane expert himself, Trenberth has been grandstanding for a decade to push a climate of fear. After the devastation of Hurricane Katrina, Trenberth convened a press conference to leverage human suffering and blame global warming. Trenberth’s ill-informed bias resulted in hurricane expert Dr. Landsea’s resignation from the IPCC. As Trenberth acknowledged he purposefully convened the press conference to counter publicized reports by hurricane experts that there was no link to global warming. Trenberth defended his conference as necessary to “correct many very misleading and erroneous reports that global warming had nothing to do with the hurricanes in recent times.”  Yet evidence of Trenberth’s links to global warming still remain elusive.

In contrast Landsea’s IPCC resignation stated, “It is beyond me why my colleagues would utilize the media to push an unsupported agenda that recent hurricane activity has been due to global warming. Given Dr. Trenberth’s role as the IPCC’s Lead Author responsible for preparing the text on hurricanes, his public statements so far outside of current scientific understanding led me to concern that it would be very difficult for the IPCC process to proceed objectively with regards to the assessment on hurricane activity.”  Yet here again Trenberth 2015 continues to mislead the public suggesting a storm like Sandy put “a potential price tag of human climate change in this storm in the tens of billions of dollars.”

Does Global Warming Really Increase Snowfall???

So why did more snow accumulate and glaciers advance during the cold of the Lttle Ice Age??

Trenberth’s 1999 paper framing the effects of global warming on extreme precipitation declared, “With higher average temperatures in winter expected, more precipitation is likely to fall in the form of rain rather than snow, which will increase both soil moisture and run off, as noted by the IPCC (1996) and found in many models.” The  2001 IPCC 3^rd Assessment repeated those expectations stating, “Northern Hemisphere snow cover, permafrost, and sea-ice extent are projected to decrease further.” Soon climate scientists like Dr. Viner proffered alarming scenarios that ‘children would no longer know what snow was’. Similarly in 2008 politicians like RFK Jr. warned DC children would be deprived of the fun of sledding due to global warming. But our climate naturally oscillates and by early February of 2010 Snowmageddon was blanketing the USA’s eastern seaboard with record snows, making global warming predictions the butt of many jokes. The heavy snows didn’t disprove CO2 had caused any warming, but it definitely highlighted failed predictions.

In 2011 Chris Mooney writing for the DeSmog blog noted heavy snowfall had become a “communications nightmare” for global warming theory and urged, “We need to move the public to a place where drawing a warming-snowstorm connection isn’t so challenging”. Kevin Trenberth was already on point. Just two weeks after the 2010 Snowmageddon, Trenberth appeared in a NPR interview flip-flopping to a new climate change framework in which a “Warming Planet Can Mean More Snow”. Now he argued, "The fact that the oceans are warmer now than they were, say, 30 years ago means there's about on average 4 percent more water vapor lurking around over the oceans than there was, say, in the 1970s”. Thus “you can get dumped on with more snow partly as a consequence of global warming," A year later the Union of Concerned Scientists held a press conference asserting global warming was no longer causing less snow, but causing heavier snow. And now, every year as heavy snowstorms approach, Trenberth and his well-groomed media outlets bombard the public, urging them not to be misled by their senses, but trust that cold and snowy days have worsened due to global warming.

Trenberth bases his warmer-earth-more-cold-and-snow alchemy on the Clausius–Clapeyron relation stating, “the water holding capacity of the atmosphere goes up exponentially at a rate of 7% per degree Celsius.” Indeed the Clausius–Clapeyron relation is undeniable physics. The problem is Trenberth misapplies it. First as seen in the graph below from the peer-reviewed paper Weather And Climate Analyses Using Improved Global Water Vapor Observations, there is little evidence of a steady increase in total precipitable water vapor (TPW) ever paralleling rising CO2. The important question Trenberth never asked was, “if TPW has declined since 1998, has there been no warming since 1998?” Indeed in accord with less water vapor, several top climate scientists have reported a global warming hiatus over the same period and the Climate Reference Network reports no warming trend over the USA for the past decade. Furthermore, ocean temperatures were in agreement. Based on Argo data a consensus of scientists reported heat content in the upper 300 meters of the ocean had “increased from 1984 to 1992 followed by a short cooling episode in 1992/93, and then increased from 1994 to 2003/2004, followed by flattening or a decrease.” Note the decline in water vapor from 1992 to 1994 and the decline since 1998 coincides with those ocean temperatures. All things considered, the uptick in heavier snow since 2009 cannot be explained by Trenberth’s new normal “warmer and wetter” assumption.

“Old school” scientists seek to understand causes of extreme events by examining changes in atmospheric circulation and other contributing weather dynamics. In contrast Trenberth does not want scientists to use the standard null hypothesis to test if CO2 warming was a contributing factor. He simply assumes CO2 must be and accuses other researchers of erroneously accepting the standard null hypothesis indicating no effect from rising CO2 (type 2 errors). Based on pure assumptions, he wants to allot some portion of every extreme event to rising CO2, even when an no anthropogenic signal emerges from standard scientific analyses and modeling experiments, as discussed in part 1.  According to Trenberth, due to the dominating effects of natural variability, CO2-driven climate models do a very poor job of simulating large changes in atmospheric circulation. While one model run will force large changes, the next model run will not. To side step that problem, instead of asking if there have been trends in atmospheric and oceanic circulation changes that produced snowfall extremes, Trenberth wants researchers to simply ask, “Was it [snowfall] related to higher than normal SSTs off the coast or farther afield” and then assume those higher temperatures were partly due to rising CO2. But that’s bad science. Higher than normal sea surface temperatures often have no connection to any theoretical CO2 heating. Warmer sea surface temperatures associated with a storm can be solely caused by a redistribution of warm water during an El Nino event. A shift in the North Atlantic Oscillation, or a shift in the jet stream can reduce wind fields and warm sea surfaces because weaker winds ventilate less heat and reduce evaporative cooling. Elsewhere shifts in atmospheric circulation can reduce cloudiness and increase solar heating.

  

Global Water Vapor trend from Vonder Haar 2012 

Trenberth has reported that 70% of the moisture involved in a storm is typically in place at the beginning of the storm, suggesting global warming has increased the available moisture. But again observations do not support Trenberth’s simplistic “warmer and wetter” attributions. For example in the 2011 Groundhog Day Blizzard the amount of available water vapor was far below normal as seen in the diagram posted by meteorologist Joseph D’Aleo at WUWT. So another question Trenberth’s attribution studies must ask, “where does the moisture come from for an extreme snow event when a region is not “warmer and wetter?”

Average Water Vapor During Winter Blizzards of 2011

Still there are many useful questions that can be asked to determine if the affects of climate change have exceeded the boundaries of natural variability. For example, do similar extreme snowfalls happen independently of sea surface temperatures that are warmer or cooler than normal? That question is easily answered from a historical perspective that encompasses just 100 to 150 years.  Historical extremes like the Great Blizzard of 1888 dropped very similar amounts of snow on America’s northeast, despite a very different climate background with colder ocean temperatures from the Little Ice Age and extensive Arctic sea ice. Comparing the Great Blizzard of 1888 with Snowmaggedon, higher than normal SST temperatures do not appear to be a critical factor.

To separate natural weather dynamics from climate change scientists must also establish why snowfall varies greatly over small timeframes; timeframes that are too short for CO2 to hypothetically alter ocean temperatures. As anyone having lived in New England knows, during any given winter the depth of snowfall is totally dependent on 2 crucial factors: 1) how fast the storm moves along the coast and 2) how far from the coast the storm travels. Unquestionably slow moving storms cause the most extreme precipitation events, rain or snow. For the American east coast, colder than normal temperatures south of Greenland encourage more frequent blocking ridges of high pressure, and those blocks cause storms to slow down and even stall. These “Greenland blocks” were also responsible for Superstorm Sandy’s sudden shift back towards the coast.

Greenland blocks are more common during negative phases of the North Atlantic Oscillation (NAO), a phase that has coincided with the recent rise in heavy snowstorms. So we must also ask if global warming has affected a shift to the negative phase of the North Atlantic Oscillation (NAO)? But previous research had suggested increased CO2 promoted a more positive NAO during the latter decades of the 20^th century. Within a framework of a single year or a few decades, shifts in the NAO are often associated changes in snowfall. But if we ask if climate change altered trends in a given NAO phase, researchers report in the paper Need for Caution in Interpreting Extreme Weather Statistics, “no significant changes either in the mean or in the entire PDFs [Probability Density Functions]” of the NAO index over the last 140 years.

As illustrated in the diagram below, the positions of cold air masses on land and warm air masses over the ocean determine where precipitation falls as snow or rain. For example during the Blizzard of 2013, despite being surrounded by warm ocean waters Nantucket Island received the least amount of snowfall (6.3 inches) while further west Providence Rhode Island (18 inches) and Hartford, Connecticut (22.8 inches) surrounded by a colder air mass received record snow. For snow to form, moist warm air must be raised to an altitude where temperatures are below freezing, with an optimal snow forming temperature hovering around -12 degrees C  (10F). Typically a cold air mass (or mountains) forces the rise in altitude. According to the Clausius-Clapeyron relation, air at 31 degree F can only hold a given amount of moisture, no matter how greatly the global average temperature varies. The critical factor that determines how much snow will accumulate is the temperature of the air nearer the ground. If lower air layers are warmer than 0°C (32F), the snow will melt as it falls forming rain, freezing rain or sleet. Only where the entire air column is below freezing do we get snow. If the storm track moves too far out to sea, or if the cold air mass is to far inland, the warm air mass gets less lift, and much less snow forms. Thus to attribute the cause of extreme snowfall a scientist must also ask, “what was the position of the storm track?” And how much cold air was in place?

Trenberth cavalierly suggests that it’s always cold enough to snow in winter, but that that is misleading. For blizzards to occur sufficient cold air must already be in place and that is not a given. Dips in the jet stream and storm tracks across North America pull cold Arctic air southward along the storms trailing edge. To produce Snowmageddon blizzards along the east coast, enough cold air had to reach the southeast and overflow the Appalachian barrier where it is dammed up along the coast (Rauber 2005). The snows that reached Jacksonville Florida in 2015 were the result of a stronger than normal flow of cold air over the Appalachians. Similar to “lake effect snow”, after flowing over the ocean, the cold dry air picked up enough moisture to dust Jacksonville with light snows.

Accordingly the National Snow and Ice Data Center experts tell us, “While it can be too warm to snow, it cannot be too cold to snow. Snow can occur even at incredibly low temperatures as long as there is some source of moisture and some way to lift or cool the air”.  In contrast, Mooney relays Trenberth’s message contradicting those experts stating, “Heavy snows mean the temperature is just below freezing, any cooler and the amount would be a lot less.”… “Warmer waters off the coast help elevate winter temperatures and contribute to the greater snow amounts. This is how global warming plays a role.”   Why would Trenberth make that up?

Dips in the jet stream and stronger storms capable of pulling an abundance of cold Arctic air equatorward are often associated with the negative phase of the North Atlantic/Arctic Oscillation (AO). Although December 2015 had been mild, when weather forecasters recognized a shift to the AO’s negative phase in early January 2016, they correctly predicted conditions would be just right for the Blizzard of 2016 that buried the mid-Atlantic States in 2 feet of snow 2 weeks later. So to explain contributions of extreme snowfall, scientists must ask how do natural cycles of the North Atlantic/Arctic Oscillation contribute to extremes.

How Air Temperature Determines Snowfall or Winter Rain

As would be predicted by a shift to more frequent negative phases of the NAO/AO, the USA was experiencing greater incursions of cold Arctic air that promoted both more record low temperatures and greater snowfall, as was the case in the 1960s and 70s. Despite projections by CO2 driven models that the ratio of record high temperatures would exceed record low temperatures by 20 to 1 in 2050, in 2013 and 2014 record low temperatures exceeded record highs. However to counter such contradictory observations, Trenberth pushes another unscientific and non-falsifiable explanation. Suggesting risingCO2 was preventing extreme cold that he claims reduce snowfall, Trenberth submitted,

“below normal temperatures can be fully consistent with climate change but are likely warmer than they otherwise would have been.”

Winter storms are low-pressure systems, or cyclones, that spin in a counter-clockwise direction as they travel across North America. Most winter cyclones in North America are initiated by the curvature of the jet stream as it passes around the Rocky Mountains, or curve northward along the eastern seaboard. The North American topography favors two major storm centers in western North America. One lies just east of the Canadian Rockies where “Alberta Clippers” form. Clippers are fasting moving storms. Typically they will not produce record heavy snowfall because the moisture supply flowing into northern North America is relatively low and the Clippers’ swift passage does not allow for sustained snow accumulation. However Clippers can evolve into major storms over the Great Lakes or eastern seaboard where moisture is available or when they align with storms initiated by the subtropical jet stream. The other storm center lies just east of Colorado. These storms often gather more moisture from the Gulf of Mexico and are slower moving. Typically these storms deliver heavier snowfall.  Of importance to east coast snowfall, either storm type will pull cold Arctic air southward and eastward toward the coast, setting the stage for greater snowfall totals from the next storm. In fact it was an Alberta Clipper that set the stage for the east coast Blizzard of 2015.

In general as illustrated below, there are 3 air masses that interact with a winter storm. 1) The cooler air that was left in place from a previous storm. This cooler air mass forces the approaching warm air to rise to altitudes where water vapor can turn to snow. 2) Warm moist air from the Gulf of Mexico or tropical Atlantic that is pulled northward by the storm’s leading edge. 3) The cold dry Arctic air pulled southward along the storms trailing edge. Mild warm conditions generated from the warm air mass typically precede a blizzard, and often catch people ill prepared for the bitter cold that follows. The most famous incident was the January 1888 School Children’s Blizzard that swept through the Great Plains. It was so named because of the 235 people who were killed, many were children who headed to school “lightly dressed because temperatures had been gradually rising to just above freezing as warm moist air was pulled up from the Gulf of Mexico. However within a few hours temperatures dropped to -29 degrees C, as the cold Arctic air advanced. Due to this counter-clockwise circulation pattern, strong storms can reverse a region’s normal latitudinal temperature gradient, temporarily making it warmer in the north and colder in the south.

How Air Masses Interact in Winter Storms

When storms track along the east coast, they intensify due to the sharp contrast between warm Atlantic temperatures and cold land temperatures. The sharp contrast favors “explosive cyclogenesis”, a phenomenon that is most common along the Gulf Stream and along the Kuroshio Current and promotes extreme snowfalls in New England and Japan respectively. In addition to the land-sea contrast, there is also a steep temperature gradient over the Atlantic due to the warm Gulf Stream. Along the coast of North Carolina in February, coastal waters are typically 10 degrees C (50 F), while just 130 kilometers to the east, Gulf Stream waters register 22 degrees C (72 F) Reddy 1994. In addition to the heat and moisture evaporating from warm Gulf Stream waters, winter storms travelling up the coast will pull warm moist tropical air northward in what is called the “warm conveyor” as illustrated below in the Washington Post illustration of the 2015 blizzard. Notice the head of the storm’s “comma” shape is an area of extreme snowfall, where the storm had pulled warm and moist air northward and westward which then rose over the colder air already in place from previous storms.

How WInter Storms Convey Warm Air and Moisture Northward

Nevertheless ignoring all the potent weather dynamics that naturally drive anomalously warmer sea surface temperatures ahead of a storm, Trenberth emailed his favorite media outlets Joe Romm, Chris Mooney and others to assert, “At present sea surface temperatures are more the 2 degrees F above normal over huge expanses (1000 miles) off the east coast and water vapor in the atmosphere is about 10% higher as a result. About half of this can be attributed to climate change.”

Was this 50% contribution ever scientifically tested and peer reviewed? Did Trenberth determine “how much warmth was transported northward on the warm conveyor side of the storm?” Did Trenberth ask how much warmth was picked up from the Gulf Stream and carried westward to cooler coastal waters? Did the storm temporarily reverse the latitudinal temperature gradient? Trenberth’s untested opinion of a 50% contribution attributed to rising CO2 was simply an opinion. It was an opinion pushed to satisfy the “need to move the public to a place where drawing a warming-snowstorm connection isn’t so challenging” and thus protect the global warming theory.

More yellow journalism followed a few weeks later in Mooney’s “What the massive snowfall in Boston tells us about global warming”. Keeping the focus on global warming Mooney reported, “sea surface temperatures off the coast of New England are flashing red”. Michael Mann added to the global warming meme reporting, “Sea surface temperatures off the coast of New England right now are at record levels, 11.5C (21F) warmer than normal in some locations.” But Mooney, Mann and Trenberth were not interested in discussing the details of those fleeting warm anomalies. They never considered the warm conveyor delivered above normal warmth northwards and then dragged that warmth and Gulf Stream warmth westward. They never tell us how fleeting those warm anomalies were. Yet for the month of February 2015 temperatures on land and sea were all several degrees colder than normal as seen in the illustration by CBSBoston’s chief meteorologist. It was extreme cold that intensified the storm. And despite below normal sea surface temperatures and thus below normal water vapor, the storm gathered enough moisture and Boston experienced record-breaking snows.

Temperaturs Were Far Below Normal During Blizzards of 2015

Trenberth has now revised his 1999 framework. Despite the record cold that reduces water vapor, he still argues global warming causes more snow in winter. He maintains warming will still cause more rain and reduced snow in the fall and spring. But again the evidence contradicts his claims. Although Trenberth focuses public attention on a decreasing trend in spring snow extent, like the winter, there has also been an increasing trend in autumn snow extent as seen in the graph below from Rutgers Global Snow Lab.

Trend in Autmn Snow Extent for Northern Hemisp
here from Rutgers Global Snowlab

So why does Trenberth persist in claiming extreme snowfalls are due to a warmer and wetter world. Trenberth betrays his intentions when he writes,  “The main way climate change is perceived is through changes in extremes because those are outside the bounds of previous weather. Climate change from human influences is difficult to perceive and detect because natural weather-related variability is large. Even with a significant climate change, most of the time, the weather is within previous bounds.” So Trenberth has organized a media campaign to not only overturn the null hypothesis, but to reverse our understanding of the difference between climate and weather. He wants you believe every extreme weather event is worsened by CO2, whether or not there is any evidence.