Many studies in North America have examined ecological and environmental correlates of the number of people affected by Lyme disease. A 2005 study using climate suitability modelling of "I. scapularis" projected that climate change would cause an overall 213% increase in suitable vector habitat by the year 2080, with northward expansions in Canada, increased suitability in the central U.S., and decreased suitable habitat and vector retraction in the southern U.S. A 2008 review of published studies concluded that the presence of forests or forested areas was the only variable that consistently elevated the risk of Lyme disease whereas other environmental variables showed little or no concordance between studies. The authors argued that the factors influencing tick density and human risk between sites are still poorly understood, and that future studies should be conducted over longer time periods, become more standardized across regions, and incorporate existing knowledge of regional Lyme disease ecology.