Dear Roger,

This is easy to do at the country scale using the FAOSTAT online data
base (http://apps.fao.org/).  With Colleagues at Goddard Institute of
Space Studies, I looked at FAOSTAT maize time series data, using a
spectral smoothing filter to identify trends, for most countries.  Our
purpose was to characterize influence of the El Nino-Southern
Oscillation, requiring us to account for any long-term trends.  The
upward trend is pretty consistent.  We did not publish the analyses.  I
have also seen presentations of positive trends for rice from several
tropical Asian countries.  While I don't think a positive trend is
universial in the tropics, it seems pretty common for the major field
crops.  You are right about the difficulty of attributing a positive
trend to any one causitive factor.  However, crop simulation should be
able to isolate climate-induced trends (positive or negative).

I hope this helps.

Roger Rivero wrote:

 > Dear friends:
 >
 > In published literature about yield forecasting in middle
 > (temperate) latitudes it has become a tradition (so it seems to me)
 >  to decompose yields time series in a linear or nonlinear mean
 > trend plus a random perturbation that alternates in sign about this
 >  mean trend.
 >
 > The mean trend is almost always an upward one and have been related
 > to the steady progress in technology (cultivars, management, etc.)
 > while the random perturbation have usually been assigned to climate
 >  variability. This random component have been the subject of
 > forecasts methods based in statistics, longe range weather
 > forecasting, biophysical models or whatever.
 >
 > Though I don´t have enough data to make any assertion about it, I
 > would like to know if any of you have been able to documentate such
 >  an upward trend in (actual) yields for low (tropical) latitudes.
 >
 > It seems to me that a trend in yield could be not only the result of
 >  technological development but of other factors as climate change.
 > But what could be worse, an upward trend may not be present in
 > lower latitudes because of a negative impact of climate change and
 > unsustainable land management leading to progressive degradation of
 >  agricultural ecosystems in spite of technological improvements as
 > irrigation, more productive cultivars and fertilizers.
 >
 > I would like to know if some of you have been able to documentate
 > these trends in lower (tropical) latitudes.
 >
 > Roger E. Rivero Vega
 >

Regards,
--
Dr. James W. Hansen
Associate Research Scientist - Agricultural Systems
International Research Institute for Climate Prediction
P.O. Box 1000
Palisades, NY 10964-8000
USA

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