How can greenhouse gases harm us?

Greenhouse gases including #"CO"_2#, #"CH"_4#, and #"N"_2"O"# have been in the atmosphere for billions of years on Earth.

Together they have produced the natural greenhouse effect that has kept the planet around #15^@"C"#, within about #+- 5^@"C"#. Without the natural greenhouse effect, produced from greenhouse gases, its doubtful if life on Earth could have evolved.

The problem is that humans are burning more and more fossil fuels that release #"CO"_2# into the air. We are also emitting methane from natural gas pipelines and introducing new sources of #"N"_2"O"# into the air.

We are essentially putting the natural greenhouse effect "on steroids" and the average annual temperature of the planets is rising ever decade.

Scientists have warned that the greenhouse effect should not climb higher than #2^@"C"# above pre-industrial levels or else we will be in serious trouble as the climate and weather start to react to a warming world. Trouble is, we have already warmed up the planet by about #1^@"C"# so not much room left! See this related question on the greenhouse effect and global warming.

The impacts of a warming world are: more wild weather to come, melting the big ice sheets that will flood many coastal cities, disruption to food supplies, more intense heat waves, more droughts, and in general, more lives lost. You can read about the scary example of Bangladesh here and the Alaskan village that already needs to be relocated here.

There are also indirect ways in which global warming will harm humans. For example, species will go extinct that may have been useful to humans or were critical to an ecosystem. Other species may spread to areas that were previously not suited for them and may act as pests, destroying crops or spreading disease.