The simplist/easiest way to change the global climate is by altering the ocean currents. A “small” dam across the Bering straight could be built. Open the gates when the tide comes in. Close them when it goes out. Or visy-versy.
Of course this is going to absolutely ruin somebodys century. It will cause “local” cooling either in northwestern Canada/United States. Or cooling in northern europe. Depending on which way you constrain the flow.
This could help rejump start the conveyer belt though.