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Climate change doubled the chances of Louisiana heavy rains, scientists warn

Posted on 16 September 2016 by Guest Author

This is a re-post from Carbon Brief by Roz Pidcock

Torrential rains unleashed on south Louisiana in August were made almost twice as likely by human-caused climate change, according to a quick-fire analysis released just weeks after the flood waters subsided.

The team of scientists concluded that such an event is expected to occur a minimum of 40% more often now than in 1900, but their best estimate is that the odds have now halved.

Dr Friederike Otto, a senior researcher in extreme weather and attribution in the Environmental Change Institute at Oxford University, who wasn’t involved in the research, tells Carbon Brief:

“It is a very striking example of the impact that climate change already has on us today…it is the rainfall event with the highest increase in risk that has been analysed, that I’m aware of.”

The new research is the latest in what are known as “single event attribution” studies. This one is notable for being the first collaboration between scientists at the World Weather Attribution (WWA) project and the US National Oceanic and Atmospheric Administration (NOAA).

A view from an MH-65 Dolphin helicopter shows flooding and devastation in Baton Rouge, Louisiana, 15 August 2016

A view from an MH-65 Dolphin helicopter shows flooding and devastation in Baton Rouge, Louisiana, 15 August 2016. Credit: Melissa Leake/US Department of Agriculture.

Historic rains

On 10 Aug 2016, a low pressure system swept into south Louisiana from the Gulf of Mexico. A combination of unusually warm water providing extra “fuel” for the storm and its sluggish movement meant it dumped a huge amount of rain in one area for several days in a row.

The WWA team said in a summary accompanying their findings:

“As the slow-moving storm crept west, it sucked up tremendous amounts of tropical moisture from the Gulf of Mexico. Downpours increased as the storm moved closer to Louisiana.”

The Baton Rouge area saw record breaking rainfall, receiving nearly 30cm in a single day on 12 Aug. Other parts of the state saw more than 75cm over the course of a week. WWA explains:

“The unnamed storm would eventually shower Louisiana with an estimated 7.1 trillion gallons of water – more rain than Hurricane Isaac in 2012, and three times as much rain as Hurricane Katrina in 2005.”

The rains and subsequent flooding in and around Louisiana claimed 13 lives and damaged more than 60,000 homes, leading the Red Cross to declare it the worst natural disaster in the United States since Superstorm Sandy hit the east coast in 2012.

Total three-day precipitation for August 12-14, 2016. Source: Climate Central

Total three-day precipitation for August 12-14, 2016. Source: Climate Central

Human role

As has become customary when events like these occur, many speculated about how human activity had altered the nature of the flooding and, therefore, how much had been avoidable.

The impacts of a warming climate on heavy rain are well understood. Basic physics dictates an increase in the amount of atmospheric moisture, which means we can expect prolonged rainfall events to be more intense under climate change, raising the risk of flooding.

The team behind today’s study set to work using two of NOAA’s high-resolution climate models to examine how the odds of Louisiana experiencing a storm of this size have changed between the mid 19th century, before the influence of human activity took hold, and today.

Their analysis took account of how changes in greenhouse gases, aerosols (such as soot and dust), ozone, volcanic eruptions and natural fluctuations in the sun’s activity have altered the chances of an event of this magnitude occurring.

The scientists also looked at measurements from 324 weather stations and compared how often the conditions experienced at the height of the Louisiana storm featured in the historical record.

They concluded that the three-day rainfall Louisiana experienced between 12-14 August is far more likely than before the world started industrialising. Dr Karin van der Wiel, a research associate at NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL) and the lead author on the new study, says:

“While we concluded that 40% is the minimum increase in the chances of such rains, we found that the mostly likely impact of climate change is a near [halving] of the odds of such a storm.”

The WWA scientists’ summary says:

“This confirms that a large part of these trends are due to climate change, including the long-term warming of the planet resulting from human activities such as burning fossil fuels.”

Complex picture

The study uses an identical approach to other attribution studies, such as a recent one that looked at the role of human activity in flooding across France and Germany in May 2016.

But the Louisiana rainfall was a particularly complicated case, say the scientists, because they had to take into account all the ways that extreme rainfall can occur, including hurricanes, frontal systems, and thunderstorms.

Dr Geert Jan van Oldenborgh of the Royal Netherlands Meteorological Institute, part of the research team, said:

“This was by far the hardest fast attribution study we have done, given all the different small-scale weather types that cause precipitation in the region. It was encouraging to find that our multi-model methods worked even for such a complicated case.”

Dr Heidi Cullen, chief scientist at Climate Central, the organisation that leads WWA, echoes this point, explaining why this means the team have high confidence in their results. She tells Carbon Brief:

“I think our entire team was surprised by how consistent the results were given the complexity of the event.”

The team has submitted their results to the journal Hydrology and Earth System Sciences, which means they are still subject to peer-review. But the methods underlying the findings are well established, the scientists explain.

Dr Peter Stott, head of climate monitoring and attribution at the Met Office, is more cautious. He tells Carbon Brief:

“It is impressive how the authors have been able to make an analysis using sophisticated climate models and techniques in such a short period of time…However this very speed from analysis to press release makes it difficult to assess.”

Stott suggests while the information is undoubtedly useful for insurers, policy makers, engineers and emergency managers, some users might prefer a slower delivery if it means more chance to fully assess its reliability. He adds:

“Therefore, it is probably wise to reserve judgement on this study until the scientific community has had a much longer time to react to this novel and interesting work.”

Already pushing the traditional boundaries of science and aligning more with news cycles, attribution methods are advancing all the time. Soon, scientists might be able to analyse more complex weather events in the same way, says Otto. She tells Carbon Brief:

“Partnering up with GFDL/NOAA for this event means that it might be possible in the not too distant future to do rapid attribution analysis of hurricanes as well, which is an exciting prospect.”

Given the destruction such events cause and the huge media attention they get, providing scientific evidence instead of just opinions when questions about humans’ role are being asked will be critical, says Otto. Oldenborgh agrees, telling Carbon Brief:

“Inevitably, people are asking the question on the role of climate change and the generalities that are often quoted in reply are not very helpful (such as ‘we live in a different world, everything has changed due to climate change’ or ‘these kind of extremes have always occurred’), we try to improve on this.”

Understanding all the ways we humans are altering the weather is important, but there’s an even more immediate motivation on the ground. Louisiana is an area already at risk of flooding and we will only see more extreme rainfall events as greenhouse gas emissions climb higher, Cullen reflects. She tells Carbon Brief:

“This is not just a case of being unlucky, climate change is increasing our risk.”

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Comments 1 to 4:

  1. So the question is what to do about it.  We are unlikely to reduce our carbon dioxide output any time soon and even if we achieved zero C tomorrow, there is some more warming in the pipe line.  Some measures that come to mind are:

    1/ Legislate against the construction of any more buildings within, say, 10 meters vertical elevation above sea level. 

    2/ Begin to put houses below this level on pilings or similar to get them above the flood zone.

    3/  Make sure there are beavers in every possible stream in Florida.

    4/  Get the army core of engineers out of the mississippi catchment and tear down levis which protect farm land upstream.  The Mississippi must be allowed to flood its flood plane and it must be able to start depositing silt where it originally did.

    5/ Plant and protect mangroves everywhere possible.  Floods from the sea will begin to be more and more common and when combined with a dump of fresh water, will have huge effects.

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  2. This study just adds to the understanding that irreversible climate disruption and ocean acidification and warming is under way. Sea level rise, more frequent storms, floods and droughts are just some of the predicaments that societies from farmers to urban dwellers will have to try to deal with as the available natural resources irrevocably decline. This will limit what engineers attempt to do. 

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  3. I think that statements such as 

    "Understanding all the ways we humans are altering the weather is important"

    ....are missleading because they imply that understanding climate change is complicated, when it is in fact quite simple. The experience of climate is complicated, but why it is changing is very straight forward. 

    There is one dirty little secret that cliamate science takes for granted, but form the dozens of people that I have asked no-one else knows. And they don't know this because they generally don't need to know, and because it is counter intuitive. I frame this in the question, " which is heavier, moist air or dry air?"  Invariably the answer is "Well Moist air of course".

    My doctor was the first to get it right off the cuff but a geologist friend took the damp appraoch, and I only discovered this fact a while ago. 

    Climate scientists please step in here if I have actually got this all wrong.

    So what is going on. It is beyond dispute that CO2 is retaining back scatter radiation and the retained energy is largely going into our oceans which are slowly warming. This warming of the ocean surface and the drying of land is increasing the moisture content of the air near the surface.

    That is the Global Warming part of the process.

    The Climate Change is what happens next. We all grew up knowing that warm air rises to form clouds, which it does. But what we were not taught is that moist air rises also. In fact the density difference between moist air with 10% moisture and dry air at the same temperature is 4%. It takes 35 deg C temperature difference in dry air for their to be a 10% difference in the density. So a body of warm very moist air has two causes to make it begin to rise to form a rain storm. There is nothing new about humid air, what is different in our lifetimes is the amount of moisture available to drive storms once they form, and where these bodies of moist air are forming and the volume of rain they create over short periods of time. That is the first part of the climate change effect, the second is in the amount of air that is being driven around by these increased energy flows, and that is the second part we experience as climate change in the form of weather. 


    Science must focus on the complexity of the process of climate, but the rest of us simply need to know the fundamental principle which is that more heat means more moisture which means more atmospheric circulation which means more intense rain, more wind, more heat and for a time more cold.

    The two primary mechanisms are CO2 absorbs back scatter radiation and re-emits it, and moist air rises. 

    I puzzled for a long time over why Low pressure systems (rising air) would form in Arctic regions, the knowledge that moist air rises answers that.


    So I argue that understanding Global Warming and Climate Change should not be shrouded in technical mystery, when it is in fact the product of several straight forward robust physical principles.

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  4. BilB is right that moist air is lighter less dense than dry air (because water vapor is less dense than air). The chain of processes (see goes like this: Warmer ocean, a lot more evaporation. Moister air goes up into clouds and more condensation releases more heat which intensifies storms and sucks in more moist air; and gives more rainfall. The other important part is that water vapor is a powerful greenhouse gas (because there is so much of it) and the longwave radiative effect of more water vapor roughly triples the effect of increasing CO2.

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