Explain the method scientist use to study how earth's atmosphere and climate have changed during the past million years.

 They use Ice Core data and tree rings. I know it sounds like a fortune teller reading tea leaves, but it's true. 

As for Earths Temperature over the last 800,000 years - Well we had at least two major ice ages known as Glaciation periods and at least one other interglaciation period - this is what we are in right now. The hard part is to figure out when another glaciation period will happen or if it will ever happen again. And we may never know in our life time or even or great grand kids life time. 

I hope this helps