Our mission presents a profound intellectual challenge due to the complexity, multiplicity, and interconnectedness of the climate system itself. What is the present state of our climate, and what causes it to change? Does the system reverberate chaotically and recombine into a new, different state? Or do external forcings such as alterations in the incidence of solar radiation and increased carbon dioxide levels prevail as agents for change? What is the combined effect of forces both internal and external acting simultaneously? And then there is the problem of timescales. We know from proxy measurements that the climate has lurched many times between glacial and interglacial states, but how do we separate long-term trends from short-term variations? Then, contributing to the complexity and blurring the difference between causality and coincidence, there are numerous, quasi-predictable cycles in the atmosphere and ocean, such as El Nino and the North Atlantic Oscillation, capable of changing weather conditions worldwide.
Industrialization and its demand for natural resources, continental-scale agriculture and its demand for land, and the swelling global population with its demand for everything continue to reshape the Earth. Since the dawn of the Industrial Revolution, our exhausts and effluviums have increased atmospheric carbon-dioxide levels by more than one third. These products of human activity have begun to leave their mark, and are changing our climate. What separates anthropogenic climate change from natural climate variations is its unprecedented rate of change. Not only by changing Earth’s climate but by doing so at fast-forward speed, humankind has become a true geophysical force.
It is of course insufficient for a scientist simply to say that humanity – or humanity in conjunction with other forces—has changed the world’s climate. As scientists, we need to identify the causes of change and, if possible, need to predict change. But to do so requires a degree of understanding of the climate system as a whole far greater than we possess at present. And that brings us back to our mission and the pressing intellectual challenge before us. To fulfill our mission by confronting the challenge, we will approach the problem in the light of two guiding questions:
How susceptible is the Earth system to perturbations?
What are the limits of Earth system predictability?