Heart of Ice (1994)

From the back of the book:
"In the twenty-third century the world is dying. The ice tightens in inexorable grip on the planet, squeezing the life out of its remaining inhabitants.

The source of this slow death is Gaia, the supercomputer designed to regulate the Earth's weather patterns. But Gaia is mad. The last glimmer of hope lies in the Heart of Volent, a mysterious meteorite which has already brought mutation and chaos to the globe. Can the Heart repair the damage and make of its wielder a new saviour?

The power to reshape reality is a tempting prize. The greatest heroes of the Age want the Heart, and the race is on to the abandoned city of Du-En, in the midst of the Saharan snow-wastes.

You will need all your resourcefulness to make the right alliances, outwit your opponents and attain the Heart. And then...?"