An unprecedented exploration of historical climate data strongly indicates increasing levels of greenhouse gases have the potential to lock California into drought conditions for centuries to come, according to an international research collaboration.
Western Geography professor Katrina Moser partnered with UCLA professor Glen MacDonald, John Muir Memorial Chair of Geography, and Illinois State University professor Amy Bloom on a paper examining the effects of greenhouse gases on drought in California. The paper, published last week in Nature Scientific Reports, looks at how natural climatic forces contributed to centuries-long, and even millennia-long periods of aridity in California over the past 10,000 years.
“What we were trying to do is give a long-term perspective on drought – and by long-term I mean thousands of years,” Moser said.
The team tracked California’s historic and prehistoric climate and water conditions by taking a sediment core in the Sierra Nevada Mountains. They pulled a 5-centemetre-wide, 3-metre-deep cylinder of sediment from the bottom of Kirman Lake and analyzed it in 1-centemetre sections, creating the most detailed and continuous paleo-environmental record of California ever.
As part of the study, Bloom, Moser’s former graduate student, developed a mathematical model to reconstruct drought over thousands of years using diatoms. Moser is an expert in diatoms, which are microscopic algae that have cell wall made of opaline silica.
“What we found is, that in the past, whenever we have periods of increased radiative forcing – and all that means is the amount of energy that the Earth is getting – whenever we increase that, then we see drought or aridity increases in California,” she said.
California has been in a drought since the new millennia started, Moser explained, and some have suggested the reason for this is because warming temperatures are increasing evaporation. Her team likewise looked to see if there was a connection between drought and sea surface temperatures, as the amount of precipitation California receives is tied to changes in sea surface temperatures.
“We did find when you increase radiative forcing, you see a cooling of the Pacific and increased drought in California. So what does that mean? As we continue to warm things up and increase greenhouse gasses, if we do see a cooling of the Pacific, then you might expect to see even more frequent and more persistent arid conditions in California,” Moser explained.
As long as warming forces like greenhouse gases are present, the resulting radiative forcing can extend drought-like conditions, more or less, indefinitely.
“So, it’s a warning – we don’t know for sure what will happen in the future, but our study is really a warning that you need to think about this.”
The biggest implication here is to plan for longer-term droughts than people have typically planned for in the past, Moser added. And it is a good message to conserve water as much as possible.
“It is a good wakeup call – or another wake-up call, there’s already many out there – that we are affecting climate, and there’s all kinds of things that can happen, that will happen, that we will have to deal with. It’s really just saying we need to think about the possibility of longer, much more persistent droughts.”
Implications of this study and thinking about long-term drought extend beyond what arid conditions mean for us, Moser said. We need to consider the implications, not only for humans but for ecosystems, in general.
“We’re in an Alpine area where climate changes rapidly as you go up in elevation, and that means, if we increase temperatures even a little bit, we can really change the conditions organisms are living in right now, and that can put a lot of stress on those organisms,” she said.
Long-term drought will greatly impact these ecosystems already living at the edge – ones that have already been impacted by humans in other ways, Moser continued.
“Much of water planning in the United States was initially based on the late 1800s and early 1900s, which happened to be wetter than normal. Now, people know drought is a real possibility in the southwestern United States and people will have planned for sort of 10-15 year-long droughts. They can probably do fairly well in dealing with those,” she said.
“But when you look at the duration of drought we see in geologic record, and I’m only going back 10,000 years – which seems like a long time but in geologic time it’s not – then we’re talking about droughts that are persisting centuries and nobody’s thought about that.”