Key Findings of the US Government’s Climate Science Special Report

Today, the US Global Climate Change Research Program released the Climate Science Special Report, Vol. 1 of the Fourth National Climate Assessment mandated by Congress to provide the latest scientific basis and impacts from climate change on the United States. Climate science continues to evolve, but in the direction of more significant realization of how humans have influenced the climate thus far, as well as how much more influence will come in the not to distant future.

Below are some of the headline findings provided in the rather powerful report (be prepared for a lot of INTENSE info):

    1. Earth’s average temperature has increased by 1 degree C (1.8 F) during the 1901-2016 period. This is faster than any rate known in the last 1,700 years.                 2017TempUpdate_Top10_Global_F_en_title_lg
    2. The average temperature of the contiguous United States has also increased by 1 degree C (1.8 F) during the 1901-2016 period. Satellite and surface observations are consistent in the detection of this rapid rise in temperature. With no change in the rate of greenhouse gas emissions, the CONUS is expected to experience a more abrupt average rise in temperature of 3.2-6.6 degrees C (5.8-11.9 F) between now and 2100.                                                                                 
      figure6_1
      Change in average surface temperature (annual and seasonal) for the period 1986-2016 since the period 1901-1960 (contiguous US; 1925-1960 for Alaska and Hawaii). Data from NOAA.

      figure6_8
      Projected changes in the coldest and warmest daily temperatures (°F) of the year in the contiguous United States. Changes are the difference between the average for mid-century (2036–2065) and the average for near-present (1976–2005) under the higher emissions scenario (RCP8.5). Maps in the top row depict the weighted multimodel mean whereas maps on the bottom row depict the mean of the three warmest models (that is, the models with the largest temperature increase). Maps are derived from 32 climate model projections that were statistically downscaled using the Localized Constructed Analogs technique. Increases are statistically significant in all areas (that is­­, more than 50% of the models show a statistically significant change, and more than 67% agree on the sign of the change). Data by NOAA.
    3. Temperature extremes in the United States are trending significantly toward record high temperatures over record low temperatures. This trend is expected to continue with the number of below freezing days also continuing to decline and days above 32 degrees C (90 F) continuing to rise.
      figure6_5
      Data by NOAA.

      figure6_9
      Projected changes in the number of days per year with a maximum temperature above 90°F and a minimum temperature below 32°F in the contiguous United States. Changes are the difference between the average for mid-century (2036–2065) and the average for near-present (1976–2005) under the higher scenario (RCP8.5). Maps in the top row depict the weighted multimodel mean whereas maps on the bottom row depict the mean of the three warmest models (that is, the models with the largest temperature increase). Maps are derived from 32 climate model projections that were statistically downscaled using the Localized Constructed Analogs technique. Changes are statistically significant in all areas (that is, more than 50% of the models show a statistically significant change, and more than 67% agree on the sign of the change).
    4. The global influence of natural variability is limited to small fraction of observed climate trends. Solar output and the Earth’s internal natural variability have contributed only marginally to the observed changes in the climate system over the past century. There is no convincing evidence for natural cycles in the observational record that could explain the changes in the climate system.                                                                                                                                                         
    5. Heavy precipitation events have increased across the US since 1901. The highest increase over the Northeast and the second highest increase over the Midwest.                                                                                                                                2017ClimateExtremes_Downpours_3_en_title_lg
    6. Northern Hemisphere spring snow cover, North American maximum snow depth and Western US snow-liquid equivalent have all declined since the early 20th century. At current rates of decline and assuming no change in water resource management, chronic, long-duration hydrological drought conditions are possible for portions of the United States by 2100.                                                                                                                                                                                               
    7. Global mean sea-level has risen 7-8 inches (~0.2 m) since 1900 with 3 of those inches since 1993. Relative to the year 2000 is very likely global mean sea-levels will rise up to 0.6 ft (0.18 m) by 2030, 1.2 ft (0.38 m) by 2050 and 4.3 ft (1.3 m)+ by 2100. A more rapid degradation of the West Antarctic Ice Sheet may mean physically possible sea level rise theoretically exceeding 8 ft (2.4 m) by 2100 (confidence is low on this).                                                                                                                2016StateOfClimate_SLR_en_title_lg
    8. The global ocean has absorbed more than 93% of the heat caused by global warming since the mid-20th century. The oceans have warmed by about 0.7 degrees C (1.3 F) during the 1900-2016 period. Assuming no emissions changes, warming of the oceans by an average of 2.7 degrees C (4.9 F) is expected by 2100.                                                                                                                                       2016StateOfClimate_HeatStorage_en_title_lg
    9. The global ocean continues to undergo rapid acidification because of dissolved carbon dioxide from atmospheric emissions. The rate of acidification is unparalleled in the past 66 million years (since the Cretaceous-Paleogene Impact Event). At the current rate, the pH of the global ocean may decline from its current average of 8.1 to as low as 7.8 by the end of the century. Seawater with pH <8 can be corrosive to shellfish, plankton and coral which depend on carbonate structures for their shells, backbones and skeletons. The greatest change in acidity will be in Arctic Ocean.

      figure13_5-1200
      Predicted change in sea surface pH in 2090–2099 relative to 1990–1999 under the higher scenario (RCP8.5), based on the Community Earth System Models–Large Ensemble Experiments CMIP5 (Figure source: adapted from Bopp et al. 2013 ).
    10. The Arctic is warming at a rate approximately twice as fast as the global average with a rapid decline in sea ice volume and extent since satellite observations began in 1979. At the current rate of warming, the Arctic Ocean will be effectively ice-free in the month of September by the 2040s.                       

      siv_annual_max_loss_and_ice_remaining
      Arctic Sea Ice Volume since 1979. Note gradual and accelerating collapse of sea ice volume. Arctic may fall below 1,000 cubic kilometers at some point in the month of September in as early as several years to a decade or so. This will happen when the yearly sea ice maximum and loss of what remains equal.
    11. Global warming has contributed “significantly” to ocean-atmosphere variability in the North Atlantic Ocean; as a result these changes have contributed to the observed upward trend in North Atlantic hurricane activity since the 1970s. North Atlantic hurricanes are expected to increase in intensity (maximum sustained wind potential) with increasing precipitation rates during the 21st century.                                    2017Hurricanes_Info_en_title_lg

      figure9_2-1200
      Tracks of simulated Saffir–Simpson Category 4–5 tropical cyclones for (a) present-day or (b) late-21st-century conditions, based on dynamical downscaling of climate conditions from the CMIP5 multimodel ensemble (lower scenario; RCP4.5). The tropical cyclones were initially simulated using a 50-km grid global atmospheric model, but each individual tropical cyclone was re-simulated at higher resolution using the GFDL hurricane model to provide more realistic storm intensities and structure. Storm categories or intensities are shown over the lifetime of each simulated storm, according to the Saffir–Simpson scale. The categories are depicted by the track colors, varying from tropical storm (blue) to Category 5 (black; see legend). (Figure source: Knutson et al. 2015; © American Meteorological Society).
    12. Large forest fires in the Contiguous US and Alaska have increased since the early-1980s. This increase is expected to continue with “profound” impacts on ecosystems.                                                                                                                           2016Wildfires_temp_WEST_en_title_lg

Some other findings of note:

-For the period 1901-2016, the Dust Bowl Era (mid-1930s) remains the most extreme era for heat. This is thought to be largely the result of significant land-surface feedbacks caused by precipitation deficits and poor land management leading to reduced vegetation and strong surface heating (which in turn promoted further drying and land degradation). However, we are on a path to eclipse this period in US climate history in the coming decades, particularly as colder conditions (more found in 1930s winters for example) continue to decline in a warming climate and extreme heat continues to increase.

-The Climate report explains (as has been explained in previous scientific literature) the period of so-called “global cooling” which occurred from the mid-1940s to mid-1960s: aerosol particles generated by WWII and post-war industrial production (esp. coal power plants) which reflected some solar radiation into space temporarily slowing long-term global warming, even as carbon dioxide concentration in the atmosphere continued to increase.

-The report notes that annual precipitation has decreased over the West, Southwest and Southeast, while increases have occurred over the Plains, Midwest and Northeast. They specifically mention an increase in mesoscale convective systems (organized clusters of thunderstorms which dump significant rainfall) over the Plains and Midwest since 1979. Mesoscale convective systems are expected to increase in frequency and intensity during the 21st century.

-While tornado climatology related to climate change has been difficult to understand because of the reliability of storm reports before the 1990s, scientists involved in the report have concluded one interesting aspect…there is moderate confidence in a decrease in tornado days (day when tornadoes of any number are confirmed), as tornadoes are increasing on those days. Greater volatility in tornado occurrence year-to-year as well as a trend toward an earlier first occurrence during the year have been observed. Studies looking at the ingredients for severe storms with all modes of potential activity (tornadoes, hail, wind) suggest an increased frequency and intensity of severe storms over areas prone to them in the US in a warmer world, but confidence on details is low.

-This report concluded that observed drought and precipitation increases (1901-2016) cannot be confidently attributed human-induced global warming. The Dust Bowl Era remains the benchmark period for extreme drought conditions. However recent negative trends in soil moisture are believed to be attributable to warming temperatures. Although soil moisture projections in climate models are still considered in their “elementary” stages in the science, based on what is known, there appears to be a signal for further decreases in soil moisture over portions of the US (particularly West and Plains) by the end of this century, increasing the risk of chronic hydrological drought.

-I find the key finding #11 I listed particularly important. There has been much debate between scientists (particularly more observational minded meteorologists vs. climatologists) about whether there has been truly observable increase in N. Atlantic hurricane activity seasonally beyond the natural variability, given the limited period of reliable satellite record and intensity measurements. This statement is given MODERATE confidence given that global warming has caused increases in sea-surface temperatures, oceanic heat content and natural cycles on multi-annual and multidecadal time scales involve changes in not only these thermodynamic variables but also dynamic ones in response (vertical wind shear, position/intensity of monsoon troughs, development of tropical waves into organized TCs).

Additional Thoughts:

This slideshow requires JavaScript.

Climate change will likely be one of the most difficult challenges the world will face this century (at least). Why? Why should we care?

When it comes to effects on people (which is what people care about), at the end of the day, what matters for the livelihood of people rich or poor? Food, water, living space. If these become challenged, you get human suffering (from economic to health threats) and geopolitical problems. The potential for significant drying and increasing chronic hydrologic droughts from loss of snowpack will lead to increasing populations in demand for resources seriously straining water resources. Crops around the world will face increasing difficulties from heat stress, prolonged droughts mixed with periods of more intense heavy rainfall events. Acidification and warming may threaten marine food resources already strained by overfishing around the world. Living space will become slowly threatened by sea level rise in low-lying areas and island nations…and more readily in the coming decades…by repeated far more extreme heat waves than previously in already hot, humid environments where cooling is not readily available, and possibly by diseases as ecosystems shift to different places, along with pests (which will also impact crops potentially).

Climate change isn’t just about warming, it’s about cascading impacts on the whole of the climate system. Without a drastic global shift to a low-carbon energy sources and the advancement of technology to remove carbon dioxide from the atmosphere, we are in store for a very challenging period in human history. This isn’t worse-case/best case or any of this. This is simply the path that we are on, no over-dramatic statements nor downplaying needed or tolerated. Hopefully we via our governments make the right choices.

–Meteorologist Nick Humphrey

Advertisements

WxClimoEd Series, Post #1, Part 2: Understanding Global Climate Change Indicators

Hello Weather & Climate News readers! This post will be the first (Part II) in what will be my ongoing education article series WxClimoEd. I hope to write educational posts on various topics related to weather and climate to help enhance your understanding of various phenomena and their impact on the environment, individuals and society. These posts will present key ideas and concepts and provide occasional linked sources to further, more detailed information.

Understanding Global Climate Change (Indicators)

In Part 2 of this article series on Global Climate Change, I’ll discuss the indicators of climate change in progress on  Earth. Even without the global measurements of temperature, there are plenty of signs in the climate system that change toward a warmer world is in progress.

IMG_3897

Earth Undergoing Abrupt Climate Change

While global warming is considered to be detectable since the First Industrial Revolution (after 1750), since the latter half of the 20th century, the environment has begun to exhibit what could be considered “abrupt” changes. Among scientists who study natural abrupt change in the paleoclimate records have had some common thought on what “abrupt” means: 1) Changes in climate which can be witnessed within a human lifetime and 2) the change is very nonlinear; it far exceeds the mechanism which initiated the change in the first place (See this video presentation by Dr. White at the American Geophysical Union Annual Meeting discussing past abrupt climate change in the paleoclimate record). In the past, abrupt change usually occurred as a result of the advance or recession of ice sheets, leading to rapid change in local temperature or regional circulations (or even global distributions of precipitation or temperature patterns). Today, abrupt change is being increasingly witnessed as a result of an already unnaturally fast mechanism (rapid rise in carbon dioxide concentration, resulting in rapid rise in global temperatures relative to natural variability…multi-decadal to centennial  scales vs. multi-millennial).

Here are just some of  the abrupt changes resulting from the changing climate happening now:

  1. Decline in sea ice over the Arctic Ocean

siv_annual_max_loss_and_ice_remaining
Arctic Sea Ice Volume since 1979. Note gradual and accelerating collapse of sea ice volume. Arctic may fall below 1,000 cubic kilometers at some point in the month of September in as early as several years to a decade or so. This will begin the process toward widespread open ocean in the Arctic for a time in September.
2. Rapid increase in air temperature of the Arctic.

The Arctic (64-90N) has warmed around 3-4 degrees C since the 1881-1910 period (based on NASA data). 2-3 degrees C of warming has occurred just since the 1951-1980 period with notable warming since the year 2000. This has led to not only the rapid decline in Arctic sea ice, but the beginning the melting of land permafrost.

temps
Average air temperatures (over land and ocean) in the Arctic region, relative to 1951-1980 average. Shown is 1984 (year I was born) to 2016. Red box show rapid warming of Arctic since 2000 compared to previous decades. “315” = +3.15 degrees C anomaly. (NASA GISS)
Addendum: The rise in the average temperature of Earth as a whole can count as abrupt, as seen in the earlier graph (farther back up). Most warming has occurred since the 1970s. The top ten warmest years on record going back to 1880 have occurred since 1998 (with 1998 now the last year in the top ten from the 20th century). 2017 is expected to be the 2nd warmest year on record just slightly behind 2016.


3. Increase in Sea Surface Temperatures and Oceanic Heat Content of Global Ocean.

The average sea-surface temperature of the global ocean from 60S-60N has risen around one degree C since the 1881-1910 period. 0.5 degrees C warming has occurred since 1980. Like the global air temperature, SSTs have been most of their record warm years since the turn of the 21st century, with an accelerated pace of warming since 2000 (1.62 degrees C/century currently, compared to 1 degree C/century 1950-2000). 2017 sea surface temperatures are currently running the 2nd warmest on record (NOAA data).

ssts
Sea surface temperature anomalies 1880-2016 for various sectors of global ocean. Values on graph are in degrees F. Oceans have warmed roughly 2 degrees F/1 degree C.
As far as oceanic heat content, the oceans have accumulated over 100 zetajoules (1 x 10^23 joules) of heat energy in the upper 700 meters of the global ocean since 1993. An incredible amount of energy, with increasingly accelerated warming in the deep ocean below 700 meters since 1993.

4. Acidification of the Global Ocean.

Earth’s seawater is slightly basic (basic is ph > 7). The global average ph of the oceans has decreased from 8.25 to 8.069 since the 1750s (ph was 8.104 in the 1990s). This is caused by the oceans dissolving carbon dioxide (30-40% of carbon dioxide released by humans dissolves in the oceans). This interaction forms carbonic acid, with further chemical reactions leading to increasing concentrations of the hydronium ion (H+). This leads to a lowering of the ph. The rate of acidifcation is faster than at anytime in the past 300 million years! The rapid acidification has been more pronounced in the Arctic Ocean because of very cold water (colder water can absorb more dissolved gases). When ph falls under 8 in the coming decades (assuming no mitigation), marine life which depend on carbonate structures (shellfish, sea snails, corals, some types of plankton, etc) begin to suffer from the corrosive effects of less basic waters.

5. Sea Level Rise

rate-of-sea-level-rise-accelerating

Sea levels are rising as a result of meltwater from land and thermal expansion as oceans warm. As the atmosphere and oceans continue to warm and weaken the Arctic and Antarctic Ice Sheets, sea levels will continue to rise, with possible nonlinear positive feedbacks accelerating it. It has already accelerated since the end of the 20th century. “King Tides” have become an increasing problem because of sea level rise in the 21st century.

6. Increase in Extreme Weather and Climate Events

The end of the 20th century into the early 21st century has featured a statistical increase in extreme weather events. Climatologists usually classify “extreme” as being 4-5+ standard deviations from the mean of all events. Such increase in extreme events over the course of years means that natural variability is being dominated by global warming, and causing a continuously shifting climate pattern.

unnamed
The shift in the range of meteorological variables across the bell curve because of climate change. The curve represents the normal distribution of events with natural variability (climate teleconnections and seasonal). The small shift of the mean by climate change causes a significant increase in less common events at one tail as well as an increase in truly extreme events not previously observed in the reference climate regime. (Presentation slide by Erick Fernandes, 2015).
Extreme events include heat, flooding, rainfall rates, drought, and wildfires. All of these occurrences have been increasing the frequency and severity around the world because of climate change. In addition, there is evidence that because of the high rate of warming of the Arctic, the mid-latitude jet stream has become weaker with increased amplitude extremes, leading to short-term and longer-term patterns favorable for extreme conditions at the surface. For example, high amplitude ridges of high pressure which do not move much or reform constantly can lead to extended periods of drought and extreme heat (while other areas downstream may receive cooler temps but heavy rainfall and flooding. This is actually something that is observable on meteorological timescales. Dr. Stefan Rahmstorf discusses the increases in extreme events from climate change in a lecture HERE.


One thing I must emphasize with understanding the impacts of global climate change is that it is impacting the environments of our world now and continue to accelerate in the coming years and decades (assuming no major changes are done). Global warming…the primary force of climate change, caused by our immense release of greenhouse gasses from fossil fuels…is the dominate force behind the rate of change in climate behavior. According to the Intergovernmental Panel on Climate Change in their 5th assessment, the world should actually be experiencing anomalous COOLING right now, but instead we have warmed Earth above and beyond natural long term global temperature variability. So when people ask “Did global warming cause (insert extreme weather event)?”, it is the wrong question. Climate looks at a collection of events for a trend. What is clear is that global warming is NOW causing a statistically significant increase in extreme events and will continue to do so. There is no “new normal” but only a continuous “ramping up” of the Earth’s natural variability toward greater extremes relative to the beginning of the Industrial Revolution, with greater impacts as humanity leaves the stable global climate in existence since the beginning of civilization.

This may be something many do not appreciate, but it is factual. Human civilization has changed Earth’s climate system to the point that we as humans are turning up the “thermostat” and started a multi-centennial experiment in geoengineering. Heat, drought, flooding, rainfall rates, wildfire events, and jet stream amplitudes, as a result, have all increased significantly in just the past 30 yrs.

If the climate were a piece of music…think of Earth’s relatively short-term natural cycles as the melody and global warming as the dominating background harmony from which the melody plays over. If the harmony changes keys, the melody will respond and shift accordingly.

In Part 3, I’ll discuss the projected future impacts of climate change being actively researched (and some already happening) such as food security, human health and living space.

Potential Impacts by Tropical Storm Nate this Weekend

Tropical Storm Nate, which developed as a depression yesterday, made landfall in Nicaragua this morning and is moving over eastern Nicaragua and Honduras this evening. Very heavy rainfall and flash flooding has already resulted in 22 deaths in Nicaragua and Costa Rica.

vis0
Heavy showers and thunderstorms producing heavy rain over portions of Central America from Tropical Storm Nate this evening and into tonight. (image valid at 5:15 pm CDT).

Nate is progressing generally northward and will emerge over the Northwest Caribbean Sea late tonight where it will have an opportunity to reorganize. The waters over that region are running in the range of 84-86 degrees F (29-30 degrees C), more than sufficient for re-intensification. With that said, the inner core will likely be badly “gutted” by the mountainous terrain of Nicaragua and Honduras and with a second landfall possible Friday evening, time will likely be limited for more robust intensification. With that said, minimal hurricane strength is possible, with a lower chance that the storm may get stronger if it’s inner core can re-organize quickly Friday.

205145_5day_cone_no_line_and_wind
National Hurricane Center forecast (issued 5 pm EDT Thursday) showing a likely landfall on the Yucatan Peninsula of Mexico Friday evening and likely US impacts on the northern Gulf Coast beginning Saturday evening.

A Hurricane Watch and Tropical Storm Warning is in effect for much of the coastal Yucatan Peninsula. Again, the major threats will be from water…heavy rain and freshwater flooding and also modest (although still hazardous) storm surge and high wave action.

Potential Impacts for Central Gulf Coast of US-

While many details are still in need of being honed in for the Central Gulf Coast…it is highly likely a tropical storm or minimal hurricane will approach the region Saturday evening with landfall early morning Sunday. The biggest threats will be from water (flooding/surge) with wind producing damage from falling trees and power outages.

p120i
NWS Weather Prediction Center 5 day accumulative rainfall forecast (valid beginning 7 pm CDT Thursday) showing heavy rainfall along the track of Nate and its remnants expected, particularly Saturday afternoon into early next week. Very heavy rainfall possible in Greater New Orleans area which is prone to freshwater flooding.

Sea surface temperatures are slightly cooler along the northern Gulf Coast north of the Loop Current (82-84 degrees F/28-29 degrees C). Still more than warm enough for intensification if the system can remain over the current (a slightly farther west track may leave it over slightly cooler waters longer).

SSTs
Analysis of Sea Surface Temperatures and the Loop Current. Nate will track along the Loop Current much of its track over water, providing with fuel to re-intensify. (Analysis by Earth Nullschool).

Also, given the shear currently over the Central Gulf will relax over the next couple of days (as an area of upper-level high pressure over Texas shifts westward and weakens), Nate will have an opportunity to re-intensify over the Gulf after leaving the Yucatan Peninsula. Computer models have some variability in timing of an upper trough which will move over the US Central Plains during the day Saturday. This will ultimately influence the exact track of the center of Nate. However both deterministic and ensemble members of the various models depict a likely landfall of the center somewhere from Southeast Louisiana to coastal Mississippi/Alabama. Regardless, widespread heavy rain (particularly near and east of the center), moderate storm surge flooding and high wind conditions will be likely over the coastal areas of these states by Saturday afternoon, spreading inland Saturday night and Sunday. Tropical storm force winds (sustained 39 mph+) will likely arrive on the LA Coast Saturday evening.

 

233552_earliest_reasonable_toa_34
Earliest Reasonable Arrival Time for Tropical Storm-Force (issued 5 pm EDT). Folks along the Central Gulf Coast should have preparations for stormy conditions completed by Saturday afternoon.

Tropical cyclone watches will likely be issued for portions of Louisiana, Mississippi and Alabama late tonight or early tomorrow morning.

Climatology Update-

The Atlantic Hurricane Season is currently running above normal (1966-2010 norms in parenthesis): 14 named storms (9), 8 hurricanes (6) and 5 major hurricanes (2). In terms of Accumulated Cyclone Energy (a function of maximum sustained winds over time), 2017 ranks (as of this post) as the 6th most active season on record for the North Atlantic Basin. The average temperature of the North Atlantic Main Development region (open tropics west of Africa) exceeded 83 degrees F (~28 degrees C) for the 9th time since 2002 (had never done so in the record prior going back to 1981). The MDR is the 3rd warmest on record overall.

DK69LgTVwAAXsJm

Global Climate Change and its Potential Connection to Hurricane Activity (cited research)

Because of recent North Atlantic Hurricane Season activity…many people have questioned whether hurricanes are becoming stronger and more numerous because of climate change. In the social media universe, I’ve seen many opinionated debates within the general public, as well as meteorologists and perhaps a few sprinkling of climatologist opinions here and there. Not to mention, interesting statements from non-climate scientists. What I have not seen much, however, is any discussion of peer-reviewed research on the topic. There’s so much knowledge being gathered every year by scientists trying to answer important questions about our past, present and future. How climate change will impact regional weather and climates is one of the most important questions because of potential impacts to people, agriculture and natural resources.

I decided to do a (very brief) search of literature on science’s current understanding of climate change as it relates to tropical cyclones. I looked into both the potential connection of global warming to these events in the current climate (attribution), as well as projections for these events based on the “business-as-usual” scenario for carbon dioxide emissions, which is a high emissions scenario and steady increase in CO2 concentration. Research cited are just a sampling of what’s out there and what I looked over. Here are some themes I found interesting (takeaway statements at the end):

Climate models* appear to show a signal toward more intense (Category 4-5 Saffir-Simpson) tropical cyclones overall in the world by the latter half of the 21st century. However, there is also a potential for a downward trend in cyclone numbers in many basins (see #1-4).

The decrease in overall cyclone numbers by the second half of the century is thought to be a product of increasing vertical wind shear over tropical oceans limiting weaker storms. However, many researchers expect there to be a significant upward trend in more intense storms (Category 4-5) as the oceans continue to warm and tropical cyclone formation and track density moves poleward. So formerly less favorable sub-regions of basins may see an overall increase in cyclone activity (with more storms which will be stronger than before in those regions) and in the increasingly less hospitable regions (over the long term), storms which do form when conditions are favorable on short time scales may see cyclones which are also more intense than in years past.

As for historical conditions leading to the present…there does not appear to be a conclusive signature by global warming on tropical cyclone intensity outside of natural variability on a global scale (3-4). However, some regional signals related to frequency changes are being actively studied. 

There is some suggestion (4) based on modeling past climate change to the present time that warming (which would enhance the potential intensity for hurricanes) has been muted by the industrial production of aerosols (particulates like sulfates and nitrates), which actually reflect sunlight from reaching Earth’s surface. However, as warming continues into mid-century, its effect of trapping heat will begin to significantly exceed aerosol cooling effects leading to the more pronounced impacts on cyclone intensity stated earlier (unless CO2 emissions are significantly reduced soon). So while global warming is happening in the background, hurricane potential intensity as we currently witness it is likely still being dominated by natural cycles. (For more on climate change research into tropical cyclones, you can also see this webinar done by climate change researcher Dr. Kerry Emanuel for Climate Central).

With that said, some researchers see signs of a global warming signature associated with recent increased tropical cyclone *frequency* in sub-regions of basins. These include the far eastern portion of the North Atlantic Basin (4), close to the East Asian Coast (5), and a portion of the North-Central Pacific Basin (6). Research is still ongoing on global warming’s past and future influence on activity in individual tropical cyclone basins.

Meanwhile, there is evidence of other impacts related to tropical cyclones (and other significant weather phenomena) and climate change. These include higher rainfall rates (7) and higher storm surge related to sea-level rise from the melting polar ice sheets and thermal expansion of the oceans (8). In addition, there is some scientific evidence that tropical cyclones in recent decades have begun to intensify more rapidly because of increased ocean warming (9). And while not completely clear yet whether it is fully tied to climate change, it is known that the observed North Atlantic Power Dissipation Index (PDI) has increased significantly since the mid-1970s (10; positively correlated to sea surface temperatures) and globally, the strongest tropical cyclones in respective basins have grown stronger since 1981 (Elsner et al, 2008…not included here). Note that scientific critics point out the use of observational data with differences in quality – satellite intensity estimates and reconnaissance flights (or lack of them) – over recent decades could put some uncertainty in these results.

My thoughts? Although inconclusive, possible intensity signals may be a hint of the projected effects of climate change as PDI and high-end cyclone intensity are highly correlated to sea surface temperatures. SSTs are increasing from global warming and this would connect with what climate models suggest of future tropical cyclone activity, if these historical trends are, in fact partially related to climate change.

The Takeaways:

  1. Tropical cyclone intensity at the highest end of the scale appears likely to increase through the 21st century because of climate change, especially if human civilization does not significantly reduce greenhouse gas emissions soon.
  2. While a current climate change signal to intensity is difficult to detect and still a matter of debate, storms in recent decades appear to be intensifying faster, are capable of producing more extreme precipitation events and higher storm surges because of rising sea levels caused by ice sheet melting and thermal ocean expansion. There also appears to be some detectable changes in frequency of storms within individual basins which may locally enhance risk.
  3. Regardless of the exact changes in frequency and intensity of tropical cyclones, the risks to individuals and society because of climate change will increase into the coming decades. It will be important for people and governments to make decisions (beyond greenhouse gas emissions) related to property, coastal land use and emergency management policy to mitigate increasing tropical cyclone hazards, particularly from water (storm surge/inland flooding).

Note: It is of EXTREME importance that those with a desire to communicate climate change issues try to inform our fellow citizens to the best of our ability. Climate change is one of the important issues facing our world (the impact on the global food supply and human health may be actually of greatest importance, but rarely discussed as those aren’t “sexy” topics…). People have their thoughts on the issue based on experiences, politics, religious/spiritual beliefs, etc. However, at the end of the day, we must inform and connect what we know to people’s concerns and allow people to decide as they may. Without censorship (“We can’t discuss climate change right now!”) or nonsensical exaggerations (“So many hurricanes, it’s a new era of superstorms!”). Stay informed (give informed opinions) and tell people why they should care as it relates to their lives. Like everything else we should communicate to the concerns of people. Considering most Americans are now, in fact, concerned about climate change, there’s really NO excuse not to discuss the issue in a serious, informed manner if we have the interest to discuss it at all.


Additional Note: *-Climate models are not weather forecast models. They do not forecast the atmosphere using initial conditions, but take a climate state (for example, our current climate) and adjust “forcings” on the climate system (carbon dioxide emissions for example). The effect of these changes to “boundary conditions” over time are interpreted for land, sea, the cryosphere and (for Earth System models), the biosphere. Global climate is based on thermodynamic and hydrologic balances which will look for equilibrium when changes to a part of the system are applied. (For more on climate models you can see this webinar by Research Meteorologist Keith Dixon of NOAA’s Geophysical Fluid Dynamics Laboratory for Climate Central).

References (links are PDFs):

#1 – Bell et al. (2013)

#2 – Murakami et al. (2011)

#3 – Wang and Wu. (2013)

#4 –  Sobel et al. (2016)

#5 – Cheng-lin et al. (2016)

#6 – Murakami et al. (2015)

#7 – Knutson et al. (2013)

#8 – Jevrejeva et al. (2016)

#9 – Kishtawal et al. (2012)

#10 – Emanual (2005)

—Meteorologist Nick Humphrey

Update on Hurricane Maria (2:30 pm EDT). High winds and flooding rains impacting Puerto Rico.

Hurricane Maria is beginning to emerge from the island of Puerto Rico after the center made landfall 8 1/2 hrs ago as a Category 4 storm with max winds of 155 mph (Cat 5 is 156+ so catastrophic wind speeds occurred). 

The hurricane is now a Category 3 storm with 115 mph sustained winds and gusts over 130 mph near the center. Damaging winds and torrential flooding rains will continue for the rest of the afternoon as the system continues to push out into open ocean water.


Most computer models indicate the system should remain offshore the United States as it moves north in a weakness in the upper level higher pressure field caused by the presence of Tropical Storm Jose offshore the Mid-Atlantic and southern New England.

 
The crucial timing to be rid of Maria forever will be the approach of a significant upper level trough of low pressure from the Midwest midweek next week to “kick” the dying hurricane out to sea. Most models show this connection keeping the system offshore being, however there is higher variability in the track after Monday which could bring the system closer to shore than expected. Currently, I feel direct impacts…the tropical storm force wind field and significant rain bands…will likely (66%+ probability) stay offshore. But potential variability makes the situation worth watching closely. 

Regardless, high surf and rip currents (currents which pull water offshore and make swimming dangerous) are likely by early next week. The system will also be weaker offshore the East Coast thanks to less intense sea surface temperatures and increasing vertical wind shear from mid-latitude winds.

In the meantime, direct impacts from a Cat 3-4 storm are likely for north coast of the Dominican Republic, the Turks and Caicos Islands and the southern Bahamas. Hurricane warnings are in effect for all these areas. 

–Meteorologist Nick Humphrey

Update on Hurricane Maria (9:25 am EDT). Catastrophic flash flood/violent wind event ongoing.

Maria made landfall as a powerful Category 4 storm near Yabucoa, PR with maximum sustained winds of 155 mph and minimum central pressure of 917 millibars (at 6:35 am EDT). The hurricane continues to move across the island delivering destructive hurricane force winds and torrential amounts of rain leading to massive flash flooding (including 5-7 inch/hr rainfall rates).

The storm currently has max winds of 145 mph. 

River gauges across PR are rising incredibly fast from the high rainfall rates:


Radar near time of landfall (currently offline):

Update on Hurricane Maria (6:30 am EDT)

#Maria is making landfall in eastern Puerto Rico as a Category 4 Hurricane with maximum sustained winds of 155 mph. Gusts of 160+ are likely in progress over northeast coast of PR. Catastrophic weather conditions will continue to spread across the island over the next couple of hours. 

Terrible situation for the 3.5 million people hunkering down this morning. 

PR Radar failed just before 6 am EDT. More inbound winds of 155 mph or higher as eyewall moved over radar site (which is in elevated terrain and winds likely much stronger).