CLIMATE CHANGE "FINGERPRINTS" NOW VISIBLE IN GLOBAL WEATHER EVERY DAY

Talk about thinking outside the box--

For decades now, climate scientists have worked to detect and measure the signal of human-caused climate change over time. They've revealed and warned us about changes in important variables such as atmospheric CO2, sea level and global average temperature; profound changes, but ones that take place over decades.

Climate change deniers, up to and including President Trump, have jumped on the fact that these incremental changes are swamped by much larger local daily and seasonal variability, making it difficult to link climate change to the weather people actually experience--with the fairly recent exception of super-storms or other extreme weather events that would have been extremely unlikely in the absence of global warming.

Now, a team of Swiss and Norweigian researchers has broken the mold. What would they see, they wondered, if they looked at variables such as temperature and humidity across space--the whole Earth--rather than over time. They were prompted to pursue this question by one of Trump's tweets using a major cold spell that hit much of the US to try to ridicule global warming.

"This lead to a lively debate about whether a single day could be informative at all about climate change," says climate scientist Reto Knutti, part of the Swiss-Norwegian research team. "Weather is not climate, yet we are seeing the imprint of climate change now in some single events, e.g. in the frequency of strong heatwaves. But what about the weather on an average day? Could we detect climate change in a single year, a month, a day?"

The answer they found is astonishing. Just by analyzing weather patterns across the globe they were able to detect the fingerprints of climate change on every day's weather since 2012.

Climate scientists can now detect signals of global warming in any day's weather

Image credit: NASA/MODIS

"Global weather is now in uncharted territory," says Knutti. "While locally of course we need a few decades for a signal to emerge from variability, the global information allows us to now detect climate change for any single day since early 2012."

The team used artificial intelligence (AI) to extract indicative patterns from the enormous mass of daily weather data around the globe. They could then test for the probability that the pattern for any given day could have appeared in the absence of climate change.

The daily weather patterns that most powerfully reveal the impact of climate change involve the global distribution of air temperature and humidity. Both variables were more pronounced over oceans than over land masses, and in the tropics compared to higher latitudes.

In retrospect, it may not be surprising that the cumulative heating effect of many decades of climate change is visible in the day-to-day distribution of temperatures across the globe. However, the researchers also found that they could still detect a strong climate change fingerprint in any given day's weather even without looking at temperature--simply from the pattern of atmospheric moisture around the world. "If we include humidity," says Knutti, "we can even detect climate change when we remove the global mean [temperature] signal for every single day, i.e. just by looking at the patterns."

If there was still any legitimate doubt that we humans have changed and are continuing to change the climate and the weather, this new way of charting the relationship between climate and weather should erase it.

Knutti writes, "In the last full IPCC report in 2013 we have shown that humans are the dominant cause of the long-term warming." We now demonstrate that since then we haven't had a single day of 'normal' weather globally. Yesterday was climate change, today is climate change, and tomorrow will be as well. We think this is a powerful message."

We couldn't agree more. The question is, is it powerful enough to get through to the deliberately deaf ears of our leaders?

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You can access the Nature Climate Change article describing this research here.

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REA

SCIENTISTS CAN NOW LINK WEATHER EXTREMES TO CLIMATE CHANGE

Climatologists have traditionally been leery of linking a specific event--an extended drought, an exceptional run of hurricanes, massive flooding--to climate change. The most they could be coaxed to say was something along the lines that climate change might have increased the odds of the extreme event, often with a comparison to rolling loaded dice or to an athlete on steroids.

Recently, however, some researchers have started to be more specific. For example, Frederika Otto, a climate scientist at Oxford University, and her colleagues recently reported that South Africa's three-year mega-drought, during which Cape Town came nerve-wrackingly close to running out of water, was demonstrably three times as likely as it would have been without human-caused climate change.

This new capability goes beyond just pinning down the increased odds of an extreme event. In three cases recently, researchers have felt confident enough to say that certain events simply could not have happened without human climate forcing. These included devastating heatwaves in Asia in 2016, the global heat record set the same year, and shockingly high sea temperatures in the Gulf of Alaska and the Bering Sea between 2014 and 2016. 

A review of 170 similar attribution studies between 2004 and 2018 found that two-thirds of the extreme weather events studied were more likely or more intense because of human caused climate forcing.

 South Africa's Theewaterskloof Reservoir, nearly empty on March 11, 2018

Credit: Zaian

So the question is, what's changed? Why are climate scientists now able to make such specific attributions of the impacts of human-caused climate change?

In principle, the answer is remarkably simple. Researchers use multiple state-of-the-art computerized climate models to run thousands of simulations with slightly different initial conditions. Some of the simulations run with current levels of greenhouse gas levels while others use pre-industrial levels. The researchers can then see how many times a comparable event shows up in each scenario. A drought as long and intense as the one that drained Cape Town's reservoirs to the brink of catastrophe happened three times more often under current atmospheric greenhouse gas levels than in models using pre-industrial levels.

In practice, it's a bit more challenging. The multiple computer simulations that are needed are costly and need a lot of time even on the fastest supercomputers. Otto and her colleagues got around this by tapping into weather@home, a network of thousands of volunteers who offer up time on their computer to help perform these massive calculations. In addition, current climate models are not fine-grained enough to model smaller events, such as a crop-destroying hailstorm or an outburst of tornadoes.

Still, scientists now have what it takes to specify to what extent many extreme events, such as a drought, massive flooding or a killer heat wave, can reliably be attributed to human-caused climate change. We can expect much more frequent, accurate and meaningful reports of this kind in the near future.

These attribution studies are not likely to convince dyed-in-the-wool climate change deniers, but they are already proving helpful to planners at all levels. For example, Helen Davies, with the Western Cape's Department of Economic Development and Tourism, now knows that the devastating drought was not a once-in-a-lifetime event. "This is an incredibly strong message which we cannot afford to ignore. We may need to work on a radically new approach to water management."

Otto hopes that this approach will soon be fast and accurate enough to be linked to extreme weather events in real time. Germany may become one of the first countries to do this on a regular basis. 'It’s part of our mission to illuminate the links between climate and weather,” says Paul Becker, Vice President of the German Weather Agency. “There is demand for that information, there is science to provide it, and we are happy to spread it.”

One example of real-time attribution is the assessment by scientists at the Lawrence Berkeley National Laboratory that Hurricane Florence is 50 miles wider and will inundate the Atlantic Coast with 50 percent more rain than a comparable hurricane before human-caused climate change.

And a new study using the supercomputers at the Lawrence Berkeley National Laboratory was able to detail the additional rainfall provoked by global warming in 15 hurricanes that took place in the last ten years, and predict even more rainfall and more intense winds if warming continues.

Let's hope that the entrenched climate change denial on the part of the current US president and congress does not keep this important emerging science from being used here at home.

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The Nature article from which most of this information comes can be found here.

The article summarizing 170 different studies, including a useful interactive map, can be found at this URL.

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