What is the downside to global warming?

There aren't any "upsides".

Potential Impacts:

1. The sea levels may rise. Technically this may be caused by melting ice from the polar ice caps or an increase in sea height as the less dense water is less compressed. Low lying land like many Pacific Islands and Miami will be under water.

2. Serious climate changes may result from diverted ocean currents. As an example, the Gulf Stream keeps Europe warm. If the Stream is not running Europe may be much cooler and drier. Crops may not grow.

3. Ecology changes on the land could include species of animals and plants extending their range to the polar regions. Local flora and fauna may become extinct. A good example is the spread of Africanized bees and Fire Ants. Sea life may also be impacted both through the introduction of competitive species presently kept in other areas by water temperature changes, and by failure to adapt to less saline water.

4. Human society may be disrupted if changes in rainfall and temperature cause crop failure. Famine refugees may require significant humanitarian attention by the "lucky" nations.