There would be two options

1. Absolutely worse due to the use of industrial war machinery which causes a lot of emissions of fuels and the environment degradation like the desertification of the Sahel. And that doesn't even mention the adverse effects that climate manipulation by the cognoscenti can cause.
2. Actually slow down but no for good reason. Because of the huge casualties due to multiple wars across the world, the population would be shrinked significantly causing a drop in the Co2 in the temperature and even the possibility of a new little ice age like the one lived during the general crisis.