At the moment, scientists agree that the world's climate 1) B warmer over the past 50 years, but they disagree about the causes. Some believe that human activities 2) ___ climate change. They argue that for 1,000 or 2,000 years before 1850, when records3)___, the temperature was more or less stable. Short warm or cold periods 4) ___ during that time, but the climate always5)___to the same level. However, since the Industrial Revolution, human beings 6) ___ more and more fossil fuels, such as coal and oil. In 1800 the atmosphere 7) ___ around 280 parts per million of carbon dioxide (CO2). Since then there 8) ___ an increase of about 3 l%. This extra carbon dioxide 9) ___ the world's temperature because of the greenhouse effect.
Other scientists disagree that human activities over the past 50 years 10) ___ global warming. They point out that volcanoes and other
natural processes 11) ___ CO2 into the atmosphere, and that human activity12) ___ a rise in CO2 of only three per cent.

In 1999, 156 countries13) ___ the Kyoto protocol, part of a United Nations agreement on climate change, which14) ___ into force in 2005.They 15) ___ to reduce their emissions of CO2 and other greenhouse gases, although so far, some countries, such as the USA and
Australia,16)___any action.
Ответы:
1. A became B has become

2. A caused B have caused

3. A began B have begun

4. A occurred B have occurred

5. A returned B has returned

6. A burned B have burned

7. A contained B has contained

8. A was B has been

9. A raised B has raised

10. A caused B have caused

11. A always released B have always released

12. A contributed B has contributed

13. A signed B have signed

14. A came B has come

15. A agreed B have agreed

16. A did not take B have not taken