It doesn't. Dr. Paul Reiter, a world-renowned expert in the field, resigned from the IPCC over that organization's insistence on making unsubstantiated claims about how global warming would spread malaria. Among other things, he said...
"...the treatment of this issue by the IPCC was ill-informed, biased, and scientifically unacceptable."
"After careful review of the pronouncements the Health chapter in Working Group II the IPCC Second Assessment, it is my opinion that that they were not based on authentic science."
"My colleague and I repeatedly found ourselves at loggerheads with persons who insisted on making authoritative pronouncements, although they had little or no knowledge of our specialty."
"..the IPCC has done a disservice to society by relying on "experts" who have little or no knowledge of the subject, and allowing them to make authoritative pronouncements that are not based on sound science. In truth, the principal determinants of transmission of malaria and many other mosquito-borne diseases are politics, economics and human activities. "