In the northern hemisphere, where most extensive occupied land surfaces are, it will become very cold. Think of a Siberian winter all the time, for many thousands of years.
On higher ground glaciers will form, coalesce and form ice caps up to a mile thick which will spread for hundreds of miles covering the most northerly lands.
South of this will be tundra hundreds of miles wide where only caribou would find enough food in summer. There would be an area either side of the equator quite suitable for life as we know it but not enough for the present human population.