|From Stiling, P. D. 1992. Introductory Ecology. Prentice Hall, Eaglewood Cliffs, NJ|
As that example shows, even small changes in average precipitation and temperature can mean major changes in communities. It's no surprise, then, to find that many species worldwide have moved toward cooler climates over the last several decades (i.e. Chen et al. 2011). Many species are also shifting their seasonal patterns (known as phenology). Plants are flowering earlier, trees are leafing out earlier, birds migrating at different times and shifting their distributions, the timing of insect life cycles is changing, and even marine plankton production is shifting (Cotton 2003, Parmesan and Yohe 2003, Root et al. 2003, Badeck et al. 2004, Edwards and Richardson 2004, Visser and Both 2005, Visser et al. 2006, Amano et al. 2010, Bartomeus et al. 2011, Zeng et al. 2011, Ellwood et al. 2013, Suikkanen et al. 2013, Wilson et al. 2013; see also IPCC 2007 chapter 18.104.22.168). This means disruptions in food webs, different tropic interactions, and changes in plant/pollinator interactions at the least although a recent experiment found few changes in plant/pollinator interactions due to climate change and suggested that pollinators may be restricting changes in plant phenology (Rafferty and Ives 2011).
The big concern is that all of these disruptions, combined with some species being unable to migrate fast enough to stay within their climate zone or evolve fast enough to adapt to new climate zones, will increase extinction rates. Thomas et al. (2004) estimated that between 15% and 37% of the species they studied will be committed to extinction by 2050. Malcolm et al. (2006) predicted that global warming would cause a mass extinction by 2050. Moyle et al. (2013) found that 100 out of 121 species of native freshwater fish in California were vulnerable to extinction due to warmer average temperatures. Böhm et al. (2013) found that 19% of all reptile species faced extinction due to a combination of factors. Foden et al. (2013) found that between 24–50% of known bird species, 22–44% of known amphibian species, and 15–32% of known coral species were highly vulnerable to climate change. Zhu et al. (2011) showed that 58% of tree species in North America are undergoing range contraction, with only 20% migrating northward. A follow-up study (Zhu et al. 2013) found that 80% of Eastern US tree species aren't migrating. That study also found that the age distribution of Eastern US tree species has shifted toward younger trees. Mature trees in Eastern North America are dying earlier and being replaced by younger trees which grow better under the warmer, wetter conditions generated by climate change.
There's also concern that such predictions are too conservative, underestimating the true extinction risk. Urban et al. (2012) showed that extinction risks were far higher in models that included dispersal rates and interspecific competition, with species with good dispersal ability overtaking and outcompeting species that had poor dispersal abilities. We also cannot count on species adapting to the new climate conditions. Quintero and Wiens (2013) found that the current pace of climate change is >10,000x faster than the pace of at which species can adapt to new climate regimes, a finding echoed by Diffenbaugh and Field (2013). Species normally adapt to new climate regimes at a rate of 1ºC per million years. The current pace of climate change over the past 30 years is 1.6ºC per century. This increased extinction risk from climate change comes on top of other known extinction threats: habitat loss, deforestation, overexploitation (i.e. Brook et al. 2003), all of which combined create far higher extinction risks than any one factor in isolation (Brook et al. 2008).
So what does that mean for us today? We're at the very start of the global warming process. While species are migrating toward the poles, the changes aren't great enough for most people to notice yet. However, within the lifetime of many people living on this Earth today, including my own children, those changes will become very noticeable, with species of trees that today are found in the subtropics ranging northward and temperate trees moving even further north (see http://www.nrs.fs.fed.us/atlas/ for what tree ranges could look like). The forests as we know them today will really no longer exist as a new mix of tree species emerges due to different dispersal rates. Many of the mammal, bird, and insect species we know today will only exist as museum specimens or in zoos, extinct in the wild. And that's a horrible legacy to leave to future generations. That makes our job today perfectly clear: Our job today is to preserve as many of those species and habitats as we can and to slow the global warming that is to come.