23 Jun We need to look at the link between extreme weather and climate change differently

A number of recent extreme weather events such as the Californian drought, the Indian heat wave, blizzards in England are prompting scientists to move beyond the traditionally stated caveat-that it is very difficult to link a single weather event and climate change-and they are now asking whether climate change played a role in making the impact of the event worse, even if it still would have occurred without climate change.

Senior Scientist with the National Centre for Atmospheric Research Kevin Trenberth urged people to assume the weather system would have occurred, and then ask how the change in the environment affected the outcome through higher temperatures, higher rainfalls, more rapid drying in drought conditions-each of which can be answered.

Trenberth and his colleagues in the Climate Analysis Section investigated thermodynamics (the interaction of moisture and temperature) from the perspective that it leads to increased evaporation of moisture, and hot air can also hold more moisture. With climate change, a growing number of scientists are of the opinion that with climate change, extreme rainfall events will be more commonas a result of a warmer atmosphere being able to hold more moisture which can fall as either snow or rain.

The research team also presented a few questions for scientists to consider in terms of understanding the relationship between a specific weather event and climate change:

• How were temperatures, rainfall and associated impacts influenced by climate change given a particular weather pattern?
• In the case of a drought, how was the drying enhanced by climate change and how did that in turn influence moisture deficits, soil dryness and the risk of wildfires? Did it increase in the intensity and duration of the drought?
• In the case of a flood, where was the moisture from? Was it enhanced by higher ocean temperatures that may have a climate change component?
• In the case of a heat wave, how was it influenced by drought, changes in rainfall and increased heat loads from global warming?
• In the case of an extreme snow event, where was the moisture from? Was it related to higher than normal sea surface temperatures off the coast or further away?
• In the case of an extreme storm event, how was it influenced by anomalous sea surface temperatures and heat content in the ocean, anomalous moisture drawn into the storm and associated rainfall and latent heating? Was the storm surge made worse as a result of higher sea levels?

Trenberth and his team are of the opinion that, when examined through the lens of the above questions, the 2010 Washington snowstorm was made worse by ocean temperatures in the tropical Atlantic 1.5°C higher than normal which funnelled an unusual amount of moisture into the storm. The also noted that Superstorm Sandy was preceded by warm ocean temperatures (partly attributable to climate change) and that human induced sea level rise contributed to the surge.

In a similar way, when examined through a thermodynamic lens, the 2013 Colorado floods are also associated with climate change according to Trenberth and his team. The rainfall event that preceded the flood originated over a section of the Pacific Ocean off the west coast of Mexico where sea surface temperatures were 1°C higher than normal, which according to Trenberth, wold probably not have occurred without climate change.

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