How should I interpret the yield changes? Expanded question: Let's take an example of India where wheat production is projected to decline by '- 2897341.18' tonnes by 2020 compared to 1990 production (ActChWHA1F2020). Since India produced '49850000' tonnes of wheat in 1990, according to this database it is likely to produce 46952658.8 tonnes ('49850000' - '2897341.18') in 2020. Since India produced '80,680,000' tonnes of wheat in 2009-10, the projected decline in 2020 is likely be 42% of the 2009-10 production. I understand that these numbers are not meant to be taken literally and are just indicative in nature. However, I want to know if I am correct in my interpretations.
The baseline is not really year 1990. It is the average yield simulated under current climate (1970-2000 baseline). Since total production is determined by many other factors beyond the effects of climate change, total production changes are not used in the assessment. The assessment is based on emissions and climate scenarios. Scenarios are devices for analyzing situations in which outcomes are uncertain. Much internally consistent assumption is built into each set of scenarios. Therefore the results cannot be interpreted as the predictions of the future. Instead, they provide the projections of the effect of climate change under a wide range of possible futures.