Climate change is speeding up the return of leaves and flowers, but by just how much?
Kate Baggaley • December 23, 2012
If you’ve read Jennifer Egan’s Pulitzer Prize-winning novel, A Visit from the Goon Squad, you’ll know that the end of the book is set sometime in the 2020s, in a futuristic New York City with 89 degree days in February and trees that bloom in January.
Scientists have been saying for years that climate change will cause briefer and more delayed snowfall seasons, together with longer and earlier growing periods for plants. And I’ve been wondering since I read Egan’s novel: Climate change may be scrambling the seasons, but trees leafing in January a mere dozen years from now? How close to reality might her vision be? How much earlier will the leaves arrive in coming years?
The U.S. Environmental Protection Agency predicts that average global temperatures will increase by 2 to 11.5 degrees Fahrenheit by 2100. But a paper that appeared in Nature this spring suggests we have been lowballing our estimates in how this might affect leafing plants.
Experiments that exposed plants to artificially warm conditions have, until now, formed our current assessment: that for each 1.8 degree Fahrenheit increase in temperature, flowering and leafing should occur between almost two and three-and-a-half days earlier. And many observations of wild plants have agreed with this, indicating that flowering and leafing shift two-and-a-half to five days earlier per 1.8-degree temperature increase.
But based on their observations of over 1,500 plant species in the wild and in experiments, the authors of the Nature study predict that in many parts of the world, leafing and flowering will actually occur a full 5 to 6 days earlier with each 1.8 degree increase in temperature.
In New York City, leafing currently occurs around early April. At this new rate, if the EPA is right and temperatures increase 2 to 11.5 degrees by 2100, leafing could occur anywhere from 6 to 38 days earlier — meaning in 2100 we’d be seeing leaves returning sometime between late February and late March.
The worst-case scenario, trees sprouting leaves in February, is pretty unnerving. But at least it won’t be as startling as the vision in Egan’s novel. A decade from now, barren trees and sweaters will still likely be the norm for February in New York City.
We are at the warm end of a long-term warming trend. It’s probably the same warming trend that ended the Little Ice Age. Most of that warming could not have been the result of increasing CO2. The period from around 1912 to around 1944 saw about the same rate and magnitude of warming that we saw from around 1975 to around 2001.
The first warming period occurred during a period with the increase in atmospheric CO2 was around one forth of that of the second warming period. At present, the concentration of CO2 in the atmospheric is around 0.04% or 400 parts per million. During the first warming period, atmospheric CO2 increased around 0.001 percentage points or by 9.6 parts per million according to the best records that we have going back that far. During the second warming period, it increased by 0.004 percentage points or by 40.1 parts per million.
There is little doubt that over the last 163 years, it has warmed on average. Over the last 12 years, at least according to the NOAA temperature record, there has been no warming even though atmospheric CO2 has increased by another 0.003 percentage points or 30 parts per million. Actually the correlation between the NOAA global temperature record and atmospheric CO2 has been good for only around 26 of the last 163 years. With the way the sun is behaving at present, we should at least consider the possibility that we could have cooling over the next several decades.