The Global Climate Change Impacts in the US report (from the US Global Change Research Program) identifies ecological thresholds that will be (or have been) crossed as a result of climate change. The effects are predicted to be widespread and deep. Water resources will be stressed (including loss of natural water storage in the mountain snowpacks of the West and Alaska), agriculture will face huge challenges of adaptation, coastal areas will be at risk from sea-level rise and storm surge, and threats to human health will increase. (In addition to climate change, we have already crossed thresholds in the rate of biodiversity loss and the disruption of the nitrogen cycle.)
The report then states the obvious: Future impacts will depend on choices we make today. The EPA's move to limit GHG emissions from the largest sources, plus its proposed limits on vehicle emissions, and the global warming legislation just introduced in the Senate are all overdue.