Announcement Of New Paper “Importance Of Background Climate In Determining Impact Of Land-Cover Change On Regional Climate” By Pitman Et Al 2011

Andy Pitman and Clive McAlpine have alerted us to a new modeling study paper that has just appeared. It is

A. J. Pitman, F. B. Avila, G. Abramowitz, Y. P.Wang, S. J. Phipps and N. de Noblet-Ducoudré, 2011: Importance of background climate in determining impact of land-cover change on regional climate. Nature Climate Change.: 20 November 2011 | DOI: 10.1038/NCLIMATE1294

The abstract reads [highlight added]

Humans have modified the Earth’s climate through emissions of greenhouse gases and through land-use and land-cover change (LULCC). Increasing concentrations of greenhouse gases in the atmosphere warm the mid-latitudes more than the tropics, in part owing to a reduced snow–albedo feedback as snow cover decreases. Higher concentration of carbon dioxide also increases precipitation in many regions, as a result of an intensification of the hydrological cycle. The biophysical effects of LULCC since pre-industrial times have probably cooled temperate and boreal regions and warmed some tropical regions3. Here we use a climate model to show that how snow and rainfall change under increased greenhouse gases dominates how LULCC affects regional temperature. Increased greenhouse-gas-driven changes in snow and rainfall affect the snow–albedo feedback and the supply of water, which in turn limits evaporation. These changes largely control the net impact of LULCC on regional climate. Our results show that capturing whether future biophysical changes due to LULCC warm or cool a specific region therefore requires an accurate simulation of changes in snow cover and rainfall geographically coincident with regions of LULCC. This is a challenge to current climate models, but also provides potential for further improving detection and attribution methods.

The paper is another contribution to examining the role of land use/land cover change on the climate system. I have just a few comments. First, they write that

“….global-scale future LULCC is probably small when compared with past changes’.

In the tropical regions, and perhaps in the boreal forests, the assumption of a small direct icrease in area of human land management of the climate system in the coming decades is not likely correct (e.g. see). In the tropical humid forests, deforestation such as in the Congo and in the Amazon are likely, unfortunately, to still  occur as population continues to grow. In semi-arid regions such as the Sahel, overgrazing is likely to expand. The boreal forest is vulnerable to not only logging, but the removal of large areas of trees by human caused fires.

My second comment is on the role of irrigation (and its areal coverage) which will also likely grow as populations demand more cropland (e.g. see). Some irrigated large areas actually use fossil water (i.e. mined from aquifers where the water has accumulated centuries ago).

Their paper concludes with the text

The need to correctly locate changes in rainfall, temperature and snow over regions of intense LULCC presents a significant challenge for climate models. The capacity of climate models to capture the background regional climate depends in part on the horizontal resolution of the model. A rigorous assessment  the relationship between climate model resolution and region simulation skill is lacking. Although finer spatial resolutions may improve global-scale simulations, how fine a model needs to be to enable reliable co-location of changes in rainfall and temperature with LULCC is unknown. Most climate models also lack many processes that might affect how LULCC affects precipitation and associated processes (see Supplementary Information). Further, there is emerging evidence that coupled ocean models are required in LULCC experiments, because these amplify the perturbation and enable effects to be captured distant from the perturbation. This suggests that, although the large-scale signal from LULCC on future climates is probably known, much higher-resolution fully coupled model simulations need to be conducted to build confidence in how LULCC interacts with a changing climate at regional scales. Our use of a coarse resolution model and fixed SSTs probably affects many aspects of our results and we are not suggesting that we have necessarily co-located changes due to CO2 with LULCC correctly. However, our main conclusion that changes in rainfall and snow caused by increases in CO2 dominate how LULCC affects climate, thereby necessitating climate models to correctly locate changes in rainfall and temperature relative to LULCC, is very probably robust.

The authors are, in my view, too optimistic regarding the skill of the large-scale multi-decadal global climate models, as has been reported frequently on my weblog (e.g. see).  Thus, their claim that “the large-scale signal from LULCC on future climates is probably known” is conjecture, as is their statement that the main conclusion that changes in rainfall and snow caused by increases in CO2 dominate how LULCC affects climate, thereby necessitating climate models to correctly locate changes in rainfall and temperature relative to LULCC, is very probably robust”. 

The dominance of CO2, that they present in their paper, remains a hypothesis. For example, they  do not include the natural and human inputs of aerosols from industrial and vehicular emissions, and also aerosols (e.g. mineral dust) from land degradation as affected by land management and land use/land cover change.  Aerosols have a diverse range of effects on the climate system even at global scales (e.g. see and see) which include major effects on precipitation processes.

Nonetheless, despite while there is an overconfidence in their conclusions,  this paper adds to our understanding of the role of land use/land cover in the climate system.

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