Scientists say that climate data from nine different countries indicate that every country will be hit with major climate changes during this century. How can scientists make such predictions?

Scientists observe data trends by observing temperatures, over long periods of time and observing related climatic changes associated with those trends, This includes winds, precipitation, cloud cover. The data is then incorporated into climate algorithms which calculated trends and impacts into the future. The predicted trends are verified as time passes to improve the algorithms to generated better trend data.

This results in future estimations of climate change.