That doesn't seem to be true. According to the WHO, climate change has the potential to cause 250,000 deaths per year in 2030-2050. When it comes to disease, malaria alone kills between 376,000 and 755,000 people per year - right now.
As one noted in the article ""almost certainly an underestimate", because a number of climate-health relationships are not measurable due to lack of adequate data".
Milder winters expand the range of disease-carrying insects, increase risks of wars or famine with less water, mass migrations leading to political instability, more extreme weather events like draughts and flooding leading to failed crops and other natural catastrophes...
Meanwhile much of the WHO estimate is for people dying due to heat exposure. Easy to quantify, important to know - but nowhere near the full picture.
Since there is a lot of uncertainty as to what the negative impact of global warming might be, doesn't it make sense to focus on the people who are dying right now?
It depends on your risk/reward curve, and your probability estimates.
Malaria is unlikely to get much WORSE than it is now. But if you think that global warming has a chance to cause catastrophic damage, two or three orders of magnitude more than malaria is causing today, then you should estimate the probability that you can impact global warming and do the EV calculation.
I suspect they did a "conservative estimate" that pops out "almost certainly at least this many people will die due to effects we understand pretty well. Other, largely independent effects could be lurking that we don't understand."
Sources: http://www.scientificamerican.com/article/premature-deaths-m... http://www.who.int/features/factfiles/malaria/en/