A、Air travel increases global temperatures five times more than car travel. B、Passenger trains cause four times more impact than

Driving alone in a car increases global temperatures in the long run more than making the same long-distance journey by air,according to a new study. However,in the short run,traveling by air has a larger adverse climate impact because airplanes strongly affect short-lived warming processes at high altitudes.
     (29) The study .published in the journal Environmental Science & Technology by a team of researchers from Austria and Norway .compares the impacts on global warming of different means of transport. In the long run,the global temperature increase from a car trip will on average be higher than from a plane journey of the same distance. However,in the first years after the journey, air travel increases global temperatures four times more than car travel. (30) Passenger trains and buses cause four to five times less impact than automobile travel for every kilometer a passenger travels.
    The research also showed that when it comes to freight transport,moving goods by planes will increase global temperatures between 7 to 35 times more than moving the same goods the same distance in an average truck. Shipping on the contrary exerts 25 times less warming in the long run,and even cools on shorter time scales.
    The study concluded that as climate change acts at various time scales.it is important to have policies to reduce both the air pollutants that have strong,short-term impacts and the long-lived gases that lead to long-term warming. (31) In addition,researchers argue that a comprehensive strategy to tackle climate change caused by the transport sector is actually to minimize the demand for transport.
29. What is the passage mainly talking about?
30. What has the study found about the impacts of different transport means in the short run?
31. What is researchers’ suggestion to tackle climate change?