1. The United Nations has (ООН в единственном числе) meetings where world leaders agree on what to do about global warming. If there is too much pollution in the air, some people have (conditional - как факт.Если будет 1-ое условие, то точно произойдет 2-ое) health problems.
2. Ecologists looked for special spots on white pine trees to know if there
were(согласование времен, множественное число) high levels of ozone.
3. The teacher asked the boys if they had done(согласование времен в косвенной речи) the task.
4.Satellite images proved that deforestation had increased (согласование времен) rapidly.