With Californians crossing their fingers in hopes of a super El Niño to help end the state's historic drought, California's water agency just delivered some startling news: for the first time in 120 years of record keeping, the winter average minimum temperature in the Sierra Nevada was above freezing. And across the state, the last 12 months were the warmest on record. This explains why the Sierra Nevada snow pack that provides nearly 30% of the state's water stood at its lowest level in at least 500 years this last winter despite precipitation levels that, while low, still came in above recent record lows. The few winter storms of the past two years were warmer than average and tended to produce rain, not snow. And what snow fell melted away almost immediately.
Thresholds matter when it comes to climate change. A small increase in temperature can have a huge impact on natural systems and human infrastructure designed to cope with current weather patterns and extremes. Only a few inches of extra rain can top a levee protecting against flood. Only a degree of warming can be the difference between ice-up and navigable water, between snow pack and bare ground.
Climate change has intensified the California drought by fueling record-breaking temperatures that evaporate critically important snowpack, convert snowfall into rain, and dry out soils. This last winter in California was the warmest in 119 years of record keeping, smashing the prior record by an unprecedented margin. Weather records tend to be broken when a temporary trend driven by natural variability runs in the same direction as the long-term trend driven by climate change, in this case towards warmer temperatures. Drought in California has increased significantly over the past 100 years due to rising temperatures. A recent paleoclimate study found that the current drought stands out as the worst to hit the state in 1,200 years largely due the remarkable, record-high temperatures.
Looking forward to El Niño, California may find soon itself at the other extreme, with global warming fueling the swing from drought to torrent. When storms do break through to California, they are now loaded with additional rainfall due to global warming, dramatically increasing the risk of flooding.
As the world heats up and more heat is carried in the atmosphere as water vapor, heavy precipitation events are becoming more intense. Like a larger bucket, a warmer atmosphere can hold and dump more water. In the past half-century, climate change has charged the atmosphere with more water vapor, fueling extreme precipitation and loading storms of all types with additional moisture that ends up as more rain and, (ironically) even more snowfall when it's still cold enough. The fingerprint of global warming has been firmly documented in the shift toward extreme precipitation already observed in the northern hemisphere.
Off the California coast recent temperatures have been 5 to 6°F warmer than historic averages --among the warmest conditions of any time in the past 30 years. In August and September California coastal buoys reached their warmest levels ever recorded.
All these record warm temperatures will help supercharge the incoming fall storms for which Californians are praying. In a warming world, it's best to be careful about what you pray for.