DescribeDescribe global warming and cooling over the past 900,000 years and during the last century. How do scientists get information about past temperatures and climates? What is the greenhouse effect and why is it so important to life on the earth? What is the scientific consensus about global temperature change during the last half of the 20th century and about projected temperature changes during this century? and cooling over the past 900,000 years and during the last century. How do scientists get information about past temperatures and climates? What is the greenhouse effect and why is it so important to life on the earth? What is the scientific consensus about global temperature change during the last half of the 20th century and about projected temperature changes during this century?