Dr. James Hansen is well known as the NASA scientist who told Congress in 1988 that global warming was already here. This is often considered to be the point at which "global warming" started to receive more attention in the press and to become better known to the public. This week Dr. Hansen was back in Washington, telling Congress 20 years later that the Earth has "long passed" the dangerous level of greenhouse gases and that we need to take drastic action to lower these levels and avoid more serious consequences (still possible, he believes, if we act quickly).
There are still some climate change skeptics, but in the past 20 years, we have collected a lot more data and have seen a growing number of problems that can be linked to climate change. Much of the data on the changes the Earth is experiencing have come from satellites operated by NASA and other space agencies, and these satellites are continuing to add to our knowledge and to allow us to see the "big picture" as polar ice caps, clouds, the level of the sea, and other things are changing. You can explore the evidence on a new NASA/JPL web site, Global Climate Change. It's a nicely designed site with excellent graphics and a wide range of interactive features.