Saturday, March 26, 2016
Tackling The Complexity of Solar-Terrestrial Climate Models
In a previous post (June 24, 2015) to do with a recent solar cooling finding, I referenced the remarkable paper, 'Solar Irradiance: Recent Results and Future Research Plans', by Thomas N. Woods of the University of Colorado. His research dealt with the solar irradiance aspect of climate change as it pertains to the Cycle 24, and in particular measurements made at the time.
Woods noted the assorted recent periods wherein irradiance measurably varied, including: the Medieval maximum, the Sporer minimum (1400s), the Maunder minimum (1600s), and the Dalton minimum (1800s). He noted with some emphasis that there is no single uniform value to characterize a time interval or period, since the irradiance itself can vary hugely on small or local scales. For example, solar flares can propel irradiance increases 50 times over normal and thereby briefly affect the radiance.
On average though, with such violent inputs smoothed out and disregarding terrestrial chemical inputs via atmospheric circulation, the Earth's temperature changes by about +0.07 K (kelvin) over a solar cycle. Compare this to the 1.6 K change (current est. increase) arising from global warming over the past 100 years mostly traced to human use of fossil fuels. Thus, the greenhouse component is nearly 23 times greater.
Even if the solar forcing on climate is enhanced by positive feedbacks the amplification is usually no more than a factor 2. So that 0.07 K increases become 0.14 K increases. The human component is still more important by a factor 11.4, a point made by Woods when he emphasized that the recent results support the hypothesis that anthropogenic greenhouse gases are the primary contributor.
The problem with Woods' research is that its connections to terrestrial climate dynamics are tenuous at best. He did not consider or factor in atmospheric circulation, the role of ozone (e.g. in absorbing UV radiation) or the generation of compounds including NO (nitric oxide) and NO2 (nitric dioxide) and their roles in O3 destruction.
In other words, his work provided a good baseline predicated on solar factors but did not provide an overall picture. Now, that is changing as we see novel improvements in solar terrestrial climate modeling (and simulations) as reported in The Journal of Advances in Modeling Earth Systems (2014, -15). The advance of these models has shown several aspects of the Sun-Earth climate system already, including:
- While the overall brightness of the Sun varies by only 0.1 %, the localized knots of magnetic energy in active regions can boost its UV output by 4-8 % at the peak of a solar cycle
- These UV rays trigger chemical reactions in the stratosphere that bind oxygen atoms and molecules to form ozone (O3). Since ozone is a good absorber of UV radiation it can heat he stratosphere near the equator which affects the winds that circle the globe.
- Increased solar activity excites the Earth's magnetic field sending high energy particles into the upper atmosphere which generates NO and NO2 that destroys ozone.
As we see from the preceding, there are often competing processes and in a complete simulation or model all must be reckoned in. One of the first such models is the 'Whole Atmosphere Community Climate Model' (WACCM) produced by the National Center for Atmospheric Research . Currently Peck et al (op. cit.) at the University of Colorado use the latest version designated WACCM4. and have benchmarked it against the earlier version WACCM3.
Fortunately, they found that the results of the new iteration are largely similar to the previous one, with just a few twists. One of them is that the atmosphere's circulation is stronger in WACCM4 which introduces twice as much NO and NO2 into the stratosphere over Antarctica, more than doubling the destruction of ozone there.
Meanwhile, the wind and temperature results are mostly consistent with the previous version. Peck et al aver that these results validate WACCM4 and lay the groundwork for a new round of studies that may refine the solar cycle's impact on terrestrial climate.
When a newer model is devised it will need to work on one long standing bias in the existing models, namely the so-called "cold pole" problem which is actually exacerbated in WACCM4. This results in stratospheric winter temperatures over the South Pole that are too low and an Antarctic polar vortex that is too strong. (However, the Arctic polar vortex - which has been wreaking havoc in the U.S. over the past few years - is significantly more accurate than the '3' version)