Irrigation increases crop yields, so the same amount of food can be grown on a smaller area. If U.S. irrigation was suddenly eliminated, making up for the lost agricultural production would require farmland expansion. Associated greenhouse gas emissions would be 363 times greater than those currently produced by irrigation itself. Emissions avoided by preventing this additional land use equal 363 years of current U.S. annual greenhouse gas emissions from irrigation.
https://source.colostate.edu/ag-irrigation-greenhouse-gas-benefits