It depends on how much the planet actually heats up. The changes associated with 4° Celsius (or 7.2º Fahrenheit) of warming are expected to be more dramatic than the changes associated with 2°C of warming.
Hotter temperatures: If emissions keep rising unchecked, then global average surface temperatures will be at least 2ºC higher (3.6ºF) than pre-industrial levels by 2100 — and possibly 3ºC or 4ºC or more.
Higher sea-level rise: The expert consensus is that global sea levels will rise somewhere between 0.7 and 1.2 meters by the end of the century if global warming continues unchecked (that’s between 2 and 4 feet). And that’s only the average. In regions like the eastern United States, sea-level rise could be even higher.
Heat waves: A hotter planet will mean more frequent and severe heat waves.
Droughts and floods: Across the globe, wet seasons are expected to become wetter, and dry seasons drier. As the IPCC puts it, the world will see “more intense downpours, leading to more floods, yet longer dry periods between rain events, leading to more drought.”
Hurricanes: It’s not yet clear what impact global warming will have on tropical cyclones. The IPCC said it was likely that tropical cyclones would get stronger as the oceans heat up, with faster winds and heavier rainfall. But the overall number of hurricanes in many regions was likely to “either decrease or remain essentially unchanged.”
Heavier storm surges: Higher sea levels will increase the risk of storm surges and flooding when storms do hit.
Agriculture: In many parts of the world, the mix of increased heat and drought is expected to make food production more difficult. The IPCC concluded that global warming of 1°C or more could start hurting crop yields for wheat, corn, and rice by the 2030s, especially in the tropics. (This isn’t uniform, however: some crops may benefit from mild warming, such as winter wheat in the United States.)
Extinctions: As the world warms, many plant and animal species will need to shift habitats at a rapid rate to maintain their current conditions. Some species will be able to keep up; others likely won’t. Coral reefs, for instance, will have difficulty adapting if the oceans continue warming and become more acidic. The National Research Council has estimated that a mass extinction event “could conceivably occur before the year 2100.”
Long-term changes: Most of the projected changes above will occur in the 21st century. But temperatures will keep rising after that if greenhouse gas levels aren’t stabilized. That increases the risk of more drastic longer-term shifts. One example: if West Antarctica’s ice sheet started crumbling, for instance, that could push sea levels up significantly. The National Research Council deemed many of these rapid climate surprises unlikely this century, but a real possibility farther into the future.