Match the items on the right to the items on the left.

Why do scientists study ice cores?	??? About 500 parts per million. How much CO2 was in the atmosphere the year that the snow fell. It traps little bubbles of the atmosphere. A thousand of years. The temperature of the year when the snow fell. They can figure out the temperatures in the past. It's a cyclical phenomenon - there was a medieval warming period too. 300 parts per million. 650,000 years. A thermometer of the temperatures in the last thousand years. The temperature gets warmer, because it traps more heat from the sun inside. Temperature and CO2 concentration.
What does snow do when it falls?	??? About 500 parts per million. How much CO2 was in the atmosphere the year that the snow fell. It traps little bubbles of the atmosphere. A thousand of years. The temperature of the year when the snow fell. They can figure out the temperatures in the past. It's a cyclical phenomenon - there was a medieval warming period too. 300 parts per million. 650,000 years. A thermometer of the temperatures in the last thousand years. The temperature gets warmer, because it traps more heat from the sun inside. Temperature and CO2 concentration.
What can scientists measure in the bubbles of the ice cores?	??? About 500 parts per million. How much CO2 was in the atmosphere the year that the snow fell. It traps little bubbles of the atmosphere. A thousand of years. The temperature of the year when the snow fell. They can figure out the temperatures in the past. It's a cyclical phenomenon - there was a medieval warming period too. 300 parts per million. 650,000 years. A thermometer of the temperatures in the last thousand years. The temperature gets warmer, because it traps more heat from the sun inside. Temperature and CO2 concentration.
What can scientists figure out from the oxygen found in the bubbles?	??? About 500 parts per million. How much CO2 was in the atmosphere the year that the snow fell. It traps little bubbles of the atmosphere. A thousand of years. The temperature of the year when the snow fell. They can figure out the temperatures in the past. It's a cyclical phenomenon - there was a medieval warming period too. 300 parts per million. 650,000 years. A thermometer of the temperatures in the last thousand years. The temperature gets warmer, because it traps more heat from the sun inside. Temperature and CO2 concentration.
How many years can scientists count back year by year in glaciers?	??? About 500 parts per million. How much CO2 was in the atmosphere the year that the snow fell. It traps little bubbles of the atmosphere. A thousand of years. The temperature of the year when the snow fell. They can figure out the temperatures in the past. It's a cyclical phenomenon - there was a medieval warming period too. 300 parts per million. 650,000 years. A thermometer of the temperatures in the last thousand years. The temperature gets warmer, because it traps more heat from the sun inside. Temperature and CO2 concentration.
How many years can scientists count back year by year in Antarctica?	??? About 500 parts per million. How much CO2 was in the atmosphere the year that the snow fell. It traps little bubbles of the atmosphere. A thousand of years. The temperature of the year when the snow fell. They can figure out the temperatures in the past. It's a cyclical phenomenon - there was a medieval warming period too. 300 parts per million. 650,000 years. A thermometer of the temperatures in the last thousand years. The temperature gets warmer, because it traps more heat from the sun inside. Temperature and CO2 concentration.
What did the scientists construct from their research?	??? About 500 parts per million. How much CO2 was in the atmosphere the year that the snow fell. It traps little bubbles of the atmosphere. A thousand of years. The temperature of the year when the snow fell. They can figure out the temperatures in the past. It's a cyclical phenomenon - there was a medieval warming period too. 300 parts per million. 650,000 years. A thermometer of the temperatures in the last thousand years. The temperature gets warmer, because it traps more heat from the sun inside. Temperature and CO2 concentration.
Which two results of the scientists' research fit together?	??? About 500 parts per million. How much CO2 was in the atmosphere the year that the snow fell. It traps little bubbles of the atmosphere. A thousand of years. The temperature of the year when the snow fell. They can figure out the temperatures in the past. It's a cyclical phenomenon - there was a medieval warming period too. 300 parts per million. 650,000 years. A thermometer of the temperatures in the last thousand years. The temperature gets warmer, because it traps more heat from the sun inside. Temperature and CO2 concentration.
What do skeptics sometimes say?	??? About 500 parts per million. How much CO2 was in the atmosphere the year that the snow fell. It traps little bubbles of the atmosphere. A thousand of years. The temperature of the year when the snow fell. They can figure out the temperatures in the past. It's a cyclical phenomenon - there was a medieval warming period too. 300 parts per million. 650,000 years. A thermometer of the temperatures in the last thousand years. The temperature gets warmer, because it traps more heat from the sun inside. Temperature and CO2 concentration.
What was the concentration of CO2 until 50 years ago?	??? About 500 parts per million. How much CO2 was in the atmosphere the year that the snow fell. It traps little bubbles of the atmosphere. A thousand of years. The temperature of the year when the snow fell. They can figure out the temperatures in the past. It's a cyclical phenomenon - there was a medieval warming period too. 300 parts per million. 650,000 years. A thermometer of the temperatures in the last thousand years. The temperature gets warmer, because it traps more heat from the sun inside. Temperature and CO2 concentration.
What is the concentration of CO2 now?	??? About 500 parts per million. How much CO2 was in the atmosphere the year that the snow fell. It traps little bubbles of the atmosphere. A thousand of years. The temperature of the year when the snow fell. They can figure out the temperatures in the past. It's a cyclical phenomenon - there was a medieval warming period too. 300 parts per million. 650,000 years. A thermometer of the temperatures in the last thousand years. The temperature gets warmer, because it traps more heat from the sun inside. Temperature and CO2 concentration.
What happens when there is more CO2 in the atmosphere?	??? About 500 parts per million. How much CO2 was in the atmosphere the year that the snow fell. It traps little bubbles of the atmosphere. A thousand of years. The temperature of the year when the snow fell. They can figure out the temperatures in the past. It's a cyclical phenomenon - there was a medieval warming period too. 300 parts per million. 650,000 years. A thermometer of the temperatures in the last thousand years. The temperature gets warmer, because it traps more heat from the sun inside. Temperature and CO2 concentration.