Posted by: ericgrimsrud | December 9, 2014

The “Not Settled” aspects of climate change science

Only in one sense is the science of global warming “not settled”. That sense concerns the additional details that are being added to the concept of anthropogenic global warming by ongoing research each year – while the basic notion of man-caused warming has not been seriously challenged in the scientific literature of the last decade. If one reads the peer-reviewed literature, examples of these new details abound. Since few within the general public read that literature, I will take this opportunity to provide just three examples of it here – gathered from research articles published in the last month. For each of these, I will include the abstract of the paper along with a brief summary of its relevance. .

One of these three papers focused on the effect of very recent volcanic activity on surface temperatures (see it at: http://www.nature.com/ngeo/journal/v7/n3/full/ngeo2098.html).

Volcanic contribution to decadal changes in tropospheric temperature
by Benjamin D. Santer et al.,
Geophysical Research Letters
Abstract
Despite continued growth in atmospheric levels of greenhouse gases, global mean surface and tropospheric temperatures have shown slower warming since 1998 than previous. Possible explanations for the slow-down include internal climate variability7, external cooling influences and observational errors. Several recent modeling studies have examined the contribution of early twenty-first-century volcanic eruptions to the muted surface warming. Here we present a detailed analysis of the impact of recent volcanic forcing on tropospheric temperature, based on observations as well as climate model simulations. We identify statistically significant correlations between observations of stratospheric aerosol optical depth and satellite-based estimates of both tropospheric temperature and short-wave fluxes at the top of the atmosphere. We show that climate model simulations without the effects of early twenty-first-century volcanic eruptions overestimate the tropospheric warming observed since 1998.

Summary by EPG: A portion of the recent slow-down in increased surface temperatures is likely due to the reflection of incoming solar radiation by the sulfate particles produced by the recent volcanic activity. (Note that individual volcanoes do not cause significant warming – they emit relatively little CO2. However, they do cause short-term cooling due to their effect on the Earth’s albedo, that is, the reflection of incoming light. This SO2-induced cooling by volcanoes only lasts a few years and is related to the size of the volcano and its ability to punch SO2 very high into the stratosphere where the removal of sulfate particulates is much slower than in the lower troposphere.)

A second recent paper (can be seen at http://onlinelibrary.wiley.com/doi/10.1002/2014GL062433/abstract) provides sme interesting insight concerning the drought in the American Southwest.

How unusual is the 2012-2014 California drought?†
by Daniel Griffi* and Kevin J Anchukaiti
Geophysical Research Letters

Abstract
For the past three years (2012-2014), California has experienced the most severe drought conditions in its last century. But how unusual is this? Here we use two paleoclimate reconstructions of drought and precipitation for Central and Southern California to place this current event in the context of the last millennium. We demonstrate that while 3-year periods of persistent below-average soil moisture are not uncommon, the current event is the most severe drought in the last 1200 years, with single year (2014) and accumulated moisture deficits worse than any previous continuous span of dry years. Tree-ring chronologies extended through the 2014 growing season reveal that precipitation during the drought has been anomalously low but not outside the range of natural variability. The current California drought is exceptionally severe in the context of at least the last millennium and is driven by reduced though not unprecedented precipitation and record high temperatures.

Summary by EPG: It appears that the drought presently being experienced in the American Southwest is unique – probably the most severe in the last millenia.

The third paper (see it at http://iopscience.iop.org/1748-9326/9/12/124002/article) examined the time delay between the emission of carbon dioxide into the atmosphere and its affect on surface temperatures.

Maximum warming occurs about one decade after a carbon dioxide emission

by Katharine L Ricke and Ken Caldeira
Environmental Research Letters

Abstract
It is known that carbon dioxide emissions cause the Earth to warm, but no previous study has focused on examining how long it takes to reach maximum warming following a particular CO2 emission. Using conjoined results of carbon-cycle and physical-climate model intercomparison projects (Taylor et al 2012, Joos et al 2013), we find the median time between an emission and maximum warming is 10.1 years, with a 90% probability range of 6.6–30.7 years. We evaluate uncertainties in timing and amount of warming, partitioning them into three contributing factors: carbon cycle, climate sensitivity and ocean thermal inertia. If uncertainty in any one factor is reduced to zero without reducing uncertainty in the other factors, the majority of overall uncertainty remains. Thus, narrowing uncertainty in century-scale warming depends on narrowing uncertainty in all contributing factors. Our results indicate that benefit from avoided climate damage from avoided CO2 emissions will be manifested within the lifetimes of people who acted to avoid that emission. While such avoidance could be expected to benefit future generations, there is potential for emissions avoidance to provide substantial benefit to current generations.

Summary by EPG: This paper suggests that CO2’s maximum effect on future temperatures occurs in only about 10 years after emission, rather than several decades as has previously been estimated. If substantiated, this new insight puts even greater emphasis on the need to cut emissions of the greenhouse gas, carbon dioxide, as quickly as possible.

Yes, indeed, the science of climate change is “not settled”, as the deniers of man-caused warming like to say. However, it is now also clear that the only scientific aspects of the issue that are “not settled” concern the endless details of the multiple factors involved. In addition, as more of these details are revealed the devils often residing within them are also exposed. From the three recent studies I have described here, we saw examples of this including: more evidence that the recent leveling of surface temperatures rise does not suggest that warming by CO2 is over; the present drought in Southern California is very likely not just part of a natural cycle; and future warming due to our emissions of CO2 will occur sooner than previously thought. I wonder how many additional details and embedded devils will have to be revealed before mankind considers the issue to be sufficiently “settled” as to merit forceful action.


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