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

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Hurricane Harvey Menacing Texas Coast

Harvey is now a Category 2 Hurricane (at 9:25 am CDT; see National Hurricane Center for latest) with max winds of 110 mph…bordering on major hurricane status…as it marches toward the coast of Texas.

Radar image out of Brownsville, TX showing Harvey wobbling west-northwest.
Visible satellite image of Hurricane Harvey showing the well-organized system as it continues to slowly intensify. It will make landfall tonight.
 

Right now the system is undergoing an “Eyewall Replacement Cycle” this occurs when a hurricane develops a secondary eyewall which bigger tha. The initial eyewall and competes with it for airflow to maintain its thunderstorms. 

Illustration of eyewall relative of the eye of a mature hurricane. In the eye dry air from the upper atmosphere is dragged down, suppressing cloud development, leading to the quieter conditions. The eyewall, in contrast, has the most violent winds in the hurricane.
Zoomed in image of Harvey showing concentric eyewall structure with “moat” – area of subsidence – between the two eyewalls. The eyewalls are wind maxima (the outer eyewall is currently stronger and the inner one may weaken soon.
 

Harvey is expected to make landfall tonight. If the ERC ends and allows for restrengthening, it may make landfall as a Category 3 storm or higher. Regardless, storm surge up to 12 ft, damaging winds and over the next few days incredible inland flooding is likely.

Harvey will sit over SE TX for days producing extremely heavy rain.
Rainfall over portions of SE TX could max out at 30-35 inches. Devastating freshwater flooding is likely in some areas of repeated rainfall as Harvey brings an air mass of very high moisture content.
 

And a blunt reminder about why you must take these storms seriously- particularly the water…


Continue to follow me on Facebook (Meteorologist Nick Humphrey) and Twitter (@wxclimonews) for more frequent updates on Harvey throughout the day today! 

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

Hello Weather & Climate News readers! This post will be the first 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 (Causes)

This article will deal with an introductory explanation of Global Climate Change as we currently understand it and the causes of it, specifically relating to human activity. Part 2 (in the next week) will discuss the known (and unknown) impacts on humanity and wildlife based on impacts on regional climates. Part 3 (in a couple weeks) will discuss mitigation efforts.

  1. Natural Climate Variability vs. Recent Global Climate Change

Earth’s climate has been evolving since the planet was born 4.6 billion years ago. These changes in Earth’s atmosphere have been largely the result of things such as biological modification of the atmosphere, volcanic eruptions, changes in ocean currents, movement of continents over tens of millions of years, variations in Earth’s orbit and other various phenomena. The most recent history of climate has featured periods of glaciation over much of the landmasses known as the Ice Ages.

Earth’s global climate has been in a “interglacial” period with a climate which warmed enough to end major continental glaciation around 11,000 yrs ago.

However, what scientists have seen in the recent climate records is a rate of change -both the climate conditions themselves and the atmospheric gases which can change the climate – at a rate accelerated and in some cases unprecedented in previous times.

203_co2-graph-021116
Carbon Dioxide (CO2) levels going back 450,000 yrs. There have been multiple periods of glaciation and interglacial periods as part of natural change (high/low CO2 = warm/cold), but CO2 has not exceeded much above 300 parts per million until the 20th century. (NASA)

What scientists in the second half of the 20th century discovered is that for the first time in the history of human civilization we are acting as a force influencing the climate. Human-induced climate change is real.

This has been found after accounting for the natural variability in climate cycles and from our understanding of how carbon dioxide works in the atmosphere. This extremely rapid change in global temperatures, a top signal of climate change, has been driven by CO2 pollution, beginning after 1850 and the during the Second Industrial Revolution.

2017EarthDay_TempAndCO2_en_title_lg
Graph showing the correlation between global temperatures (anomaly relative to the 1881-1910 average) and CO2 concentration 1880-2016. CO2 is known physically to be capable of “trapping” heat in higher concentration. (Climate Central)

One of the first questions which many non-scientists have about global climate change is “Well hasn’t the climate always been changing?” The answer is yes, but with a caveat…it has NOT changed this rapidly in the history of humanity on Earth (Homo sapiens sapiens have been around ~200,000 yrs). Since Earth began the process out of the last glaciation 22,000 yrs ago, global temperatures have varied up to a 4 degrees C. But this process of variability has occurred over the course of centuries to a couple of millennia because of the long cycling of natural processes. Human activity in the form of burning fossil fuels since the latter 19th century has caused around 1 degree C of warming within a 100-150 yr period. This comic comparing the average temperature of Earth to the end of the Ice Age and achievements of human civilization across millennia strongly illustrate the climate stability we’ve come to flourish under. (zoom in at the top and scroll down…it’s long, but worth the read and look!). Humans have essentially acted as a continuously spewing ‘volcano’ of carbon products. And as a result, we have compressed warming which would take nature nearly a millennia to do in recent geologic time and accomplished it within a century.

We are transforming our world before our eyes.

2. How does the greenhouse effect work?

The greenhouse effect is a simple concept, but one with major implications for us all in this major environmental issue. In Earth’s atmosphere exists gases which are chemically capable of “trapping” heat in the lower atmosphere, heating the surface. The three most significant “greenhouse gases” are carbon dioxide, water vapor, and methane. These gases in higher concentrations are most effective in reflecting infrared (heat) radiation originally emitted by Earth’s surface back to the surface as opposed to allowing it to be lost to space. If it wasn’t for this greenhouse effect, Earth would be literally completely glaciated over and very cold.

640px-The_green_house_effect.svg
Example graphic showing the “radiation budget” and how the greenhouse effect impacts how much radiation is retained by Earth’s atmosphere. (By Wikipedia user ZooFari)

Methane is very potent, but short-lived without constant replacement as direct sunlight breaks the molecules apart. Water vapor varies seasonally with some balance because of the water cycle. Therefore carbon dioxide or CO2 ends up being the most important changing variable for warming/cooling the climate. Such changes, along with other variable cycles, have influenced previous periods of ice ages. But human “eruption” of CO2 since the 2nd half of the 19th century has led to clear changes in Earth’s atmosphere and climate.

This YouTube video shows 131 yrs of global warming (1880-2011). The “anomalous temperature” is relative to the average planetary temperature of 1880-1910. The average temperature of Earth has continued to warm above this average to record levels since 2011. The planet is known to have warmed by ~1 degree C (nearly 2 F) since the late 19th century. The most dramatic warming (as can be seen in the video) began to ramp up significantly since 1980. Depending on the scenario, human-induced warming of Earth may reach 1.5-2 degrees C (2.7-3.6 F) above pre-industrial levels by 2050 and (depending on human efforts to decrease CO2 pollution) the warming may hit 2-4+ degrees C (3.6-7.2+ F) by 2100.

3. The importance the rate of warming

Such rapid rates of warming are what alarms climatologists and those who understand the importance of climate stability for life and society on Earth. Remember the last time you saw a wholly mammoth, saber-toothed cat, wholly rhino, or an indigenous North American horse? Never because they all went extinct from the “rapid” climate change of the final end of the ice age glaciation 11,000 yrs ago. What’s “rapid”? Temperatures warmed more than 2.5 degrees C over the course of 1500 years. We may accomplish that in 150-200 yrs! Incredible. But yet the “rapid” natural changes were not non-consequential. Many more species than listed which couldn’t adapt died out over hundreds to a couple thousand years. And minor variations in global climate by 0.5 degrees C or less – or even significant regional variations which barely showed up on the global signature – have led to major losses in agriculture and economics producing stresses on some nations (poverty, famines, wars) and in some cases, isolated civilizations simply died out. Having such rapid, unprecedented warming is a cause for concern because too many species (and many socio-economic or geographically vulnerable peoples) may face significant harm in the face of an inability to adapt to transforming climate regimes.

In Part 2, later this week, I’ll discuss the effect climate change has on regional climates and likely impacts being faced by humans and wildlife as the climate continues to heat up extremely quickly.

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Hilary and Irwin making plans for a Fujiwhara waltz over the open ocean

Let’s learn something cool about the tropics!

The past couple of weeks, the Eastern Pacific Basin has been quite active with multiple active tropical cyclones churning, dying and new ones forming. All while the North Atlantic Basin has been largely silent. We’ll get into the pattern set up for tropical activity between the two basins in the coming few weeks in a moment. But first, let’s discuss two interest systems in process in the EPac – Hilary and Irwin.

(Intensities are as of 4 pm CDT Thurs)

Hurricane Hilary is currently a Category 1 with max winds of 75 mph moving to the west-northwest. Meanwhile, ~480 nautical miles southwest of Hilary is Tropical Storm Irwin. It has max winds of 60 mph and drifting westward. Neither system is a threat to populated land masses and both are quickly heading for cooler waters north of the subtropical Pacific, where much drier air also exists in the mid-levels of the atmosphere. Both will lead to rapid deterioration of the cyclones this weekend.

vis
Visible satellite image showing Tropical Storm Irwin and Hurricane Hilary in close proximity at 4 pm CDT July 27, 2017. Cooler waters are noted to the north producing stable air mass near the sea surface, including deep marine stratus clouds and “closed cell” cumulus cloud deck. Stable air mass is non-conducive for thunderstorms hurricanes need to sustain themselves.
wv
Same image at same time as above in the “water vapor” infrared channel, where mid-level water vapor is detected. The circled area is a fairly dry area where air is experiencing sinking motions, coinciding with the cooler surface waters and the lack of thunderstorm activity for deep atmospheric moisture. Deep dry air can disrupt thunderstorm activity in tropical cyclones and weaken them.

Later Sunday and into Monday, an amazing phenomenon is expected to occur. Because of the very close proximity of Hilary and Irwin – only several hundred miles apart – the two cyclones are expected to undergo a Fujiwhara Interaction. This describes when two vortices in close proximity begin to rotate around a common center or one around another if one is more dominant. It is named after the Japanese meteorologist Sakuhei Fujiwhara who first described the interaction scientifically in a 1921 paper.

It does not happen very often, but it is typically more common in the Western Pacific basin where a very large surface area of favorable tropical development and maintenance exists and many cyclones can develop simultaneously and sometimes in close proximity.

Numerical models have shown the possible interaction for days.

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The slideshow above displays the Hurricane Weather Research & Forecasting Model forecast initialized at 11 pm PDT Wednesday with the forecast valid 5 pm PDT Sunday – 11 am PDT Monday. This model is “nested” on Hilary to show its evolution (the colors are surface temperatures in degrees C and wind barbs are in knots). However, you can see Irwin orbiting around it on its south and east sides Sunday evening – Monday morning. Irwin will likely weaken and die (along with Hilary, not long after), or will become absorbed by Hilary.

Nature never ceases to amazes in what it can do with the laws of physics!