Previous chapters have explored the varied ways in which the ocean and atmosphere interact, and the importance of this interaction to the Earth's present and past climate. The principal question in climatology today, however, is the path of future climatic change. In the 1970s the Northern Hemispheric cooling of the previous two decades, coupled with the future trend in solar forcing from orbital variation (§ 7.1.2), led to speculation that the Earth was entering the initial stages of the next glaciation. The global warming of the next two decades fuelled intense argument over the impact of anthropogenically-driven increases in the concentration of greenhouse gases. These debates have served greatly to advance our understanding of the climate system. This chapter brings together the underlying causes of present climatic change, both natural (§ 7.1) and anthropogenic (§ 7.2), and identifies the ocean's contribution to the question ‘where next?’ (§ 7.3).

Natural variability

Solar variability

The Sun is the fundamental source of energy for the Earth's climate, yet remarkably little is known about the variability of its energy output. The theory of stellar evolution implies that the Sun's radiance has increased by 30% over the 3.5 billion years during which life has existed on this planet. It has been shown (§ 6.1) that a much stronger greenhouse effect than that seen today would have been needed in the distant past to counter global glaciation.