It is said that going vegetarian can help reduce global warming. What research has been done to support this or against it. Also, if true, to what extent is it effective as compared to other methods like reducing emissions from industries, having low-polluting vehicles etc.?
There is an Oxford University study on the Dietary greenhouse gas emissions of meat-eaters, fish-eaters, vegetarians and vegans in the UK it concludes that
...an average 2,000 kcal high meat diet had 2.5 times as many GHG (higher greenhouse gas) emissions than an average 2,000 kcal vegan diet.
It concludes that
moving from a high meat diet to a vegetarian diet would reduce your carbon footprint by 1,230kgCO2e/year, and moving from a high meat diet to a vegan diet would reduce your carbon footprint by 1,560kgCO2e/year.
As a comparison for scale, an economy return flight from London to New York has a carbon footprint of 960kgCO2e according to this carbon footprint calculator.
The most extensive study and publication on this subject is 2006 FAO's report titled "Livestock's long shadow". In chapter 4 it analyzes the role of livestock industry in greenhouse gases emissions and states it's responsible for about 18% of GHG emissions, more than transportation sector. We can hence infer that stop consuming meat has higher impact than improving the technologies related to transportation.
Another study worth to be quoted is "Total Environmental Impact of Three Main Dietary Patterns in Relation to the Content of Animal and Plant Food". It states that meat consumption has a great impact on environment, including climate change, leading to similar conclusions as the previous source.
Deforestation is strictly related to GHG emissions and climate change, since destroyed forests provoke CO2 emissions (see FAO 2006 for a detailed explanation). A 2016 publication by Nature proyected several scenarios to 2050. The main differences were determined by the diet. Again, this means that leaving meat can deeply have impacts on world future and climate change.
Similar conclusions were found in another study called "Analysis and valuation of the health and climate change cobenefits of dietary change". Here researchers find that:
health and climate change benefits will both be greater the lower the fraction of animal-sourced foods in our diets.
Finally, additional reference can be found in the "facts" page of the documentary "Cowspiracy".