August 23, 2013

Heavy Rains Down Under Briefly Masked Sea-Level Rise

by Julia Rosen

Heavy rains transformed parts of Queensland between September 2009 and March 2011. (Source: NASA image courtesy LANCE MODIS Rapid Response)

Heavy rains transformed parts of Queensland between September 2009 (left) and March 2011 (right). (Source: NASA image courtesy LANCE MODIS Rapid Response)

From late 2010 to early 2011 — the summer months in Australia — it poured. In fact, that season marked the midpoint of the wettest two-year period on record for the country; huge swaths of the normally arid continent sputtered beneath an extra foot of rainfall. Catastrophic floods in the coastal province of Queensland made news headlines around the world as rivers swelled, residents fled their homes, and dozens tragically lost their lives. But heavy rains fell in other regions as well, pooling in the empty interior of the Outback. Like a bathtub, Australia’s unique geography trapped this water and prevented it from flowing out to sea. The continent retained so much water, scientists report in a new study, that it temporarily offset sea-level rise.

Measurements from tide gauges, and more recently, satellites altimeters reveal that global sea levels have risen more than 20 centimeters since the 1880s as a result of climate change. But the increase — readily apparent in the long-term record — has proceeded in fits and starts, punctuated by years like 2011 when the trend stops or even temporarily reverses. In these years, the natural variability of the climate system trumps its slower, monotonic response to steadily increasing carbon dioxide concentrations in the atmosphere. And these years provide opportunities for debate and education: scientists and the public discuss what variability means for climate change, and scientists analyze these “natural experiments” in extreme weather to test their understanding of the climate system.

Tide gauges (red and blue) and satellite altimeters (black) have tracked rising sea levels for more than a century. (Source: IPCC)

Tide gauges (red and blue) and satellite altimeters (black) have tracked rising sea levels for more than a century. (Source: IPCC)

Satellites measured the drop in sea levels that coincided with heavy Australian rains in 2010 and 2011. (Source: Cnes/CLS/Legos)

Satellites measured the drop in sea levels that coincided with heavy Australian rains in 2010 and 2011. (Source: Cnes/CLS/Legos)

It is tempting to focus on short-term decreases in the records of global sea level or surface temperature or atmospheric carbon dioxide or methane — all of which are headed up — and wonder if these instances show that climate change is slowing down, or perhaps not happening at all. If these records were only a few years long, this would be a reasonable conclusion. But zooming out to take in the full picture leaves little doubt that a hiatus lasting a year, or even a decade, does not mean the end of climate change. Pauses have happened regularly throughout the last century, and as before, plateaus and reversals will most likely give way to further increases in the future.

Taking the opposite perspective, there are many who observe extreme climate events like the Australian floods of 2010-2011 or Hurricane Sandy and ask if these unusual events were caused by climate change. Again, this is a totally reasonable question, but it is almost impossible to answer because of the inherently variable nature of the climate system itself. Rather than ask whether climate change brought on a specific event, climate scientists suggest that a better question is whether climate change will make similar events more likely. Scientists say climate change is like rolling a loaded dice: it increases the odds that an extreme event will happen, but it doesn’t mean you can’t roll ordinary ones and twos sometimes too.

On top of illustrating the naturally “noisy” behavior of the climate system, unique events like the Australian floods challenge scientists to explain them. So what happened in the Southern Hemisphere spring of 2010?


The GRACE satellites can measure small differences in gravity caused by redistribution of water across the world’s continents. (Source: Boening et al., 2012)

In the new study, led by John Fasullo at the National Center for Atmospheric Research in Boulder, CO, the scientists trolled through heaps of available data, collected routinely to record the state of the Earth’s climate, in search of clues. They noticed that just as satellites were picking up the 7 millimeter drop in average global sea levels, Australia was gaining weight. A pair of satellites known as GRACE (short for the Gravity Recovery and Climate Experiment) orbit the Earth 15 times a day, measuring small variations in the gravitational pull of the surface beneath them. When a region experiences heavy snow or rain, it gains mass and exerts a stronger gravitational force. (Just as melting glaciers transfer water from land to sea, intense rainfall can do the opposite.) Using the GRACE measurements, the scientists suspected they had found the missing water, it was hiding out in the soils and isolated lakes of central Australia.

But why did it rain so hard? In their study, to be published next month in the journal Geophysical Research Letter, the authors suggest that several different modes of natural climate variability coincided to produce the heavy rains: a strong La Nina provided a source of moisture in the Pacific Ocean, more of that moisture made it onto the continent thanks to a climate pattern known as the Southern Annular Mode, and even more moisture trickled in from the Indian Ocean due to an oscillation called the Indian Dipole that warms the water along Australia’s west coast in some years. While these modes of variability are natural, they now operate against the shifting backdrop of climate change; scientists have previously noted that the floods coincided with record high sea surface temperatures along the country’s coasts.

The weather patterns that drove the floods have broken down and reorganized since 2011, and the water cycle has returned much of this precipitation to the ocean, causing sea level to resume rising at a rate of about 10 millimeters per year. But as lead author John Fasullo notes in an NCAR press release, natural experiments like this, although dangerous and beyond anyone’s control, serve as “a beautiful illustration of how complicated our climate system is.”



Learn more about the hydrologic cycle in our module.

Check out beautiful pictures of Lake Eyre, a large dry lakebed in central Australia that filled with water during the floods.

Read more about climate variability at NASA’s science portal.

Julia Rosen

Written by

Julia Rosen is a freelance science writer and PhD student at Oregon State University. She received a Bachelor’s degree in Geological and Environmental Sciences from Stanford University before beginning her doctoral research on polar ice cores and climate change. In between, she did her “Master's” in backpacking around the world and skiing. Julia is a periodic contributor to Oregon State’s research magazine, Terra, and helps write blog content and develop learning modules for Visionlearning.