Steffen, W., Andreae, M. O., Bolin, B., Cox, P. M., Crutzen, P. J., Cubasch, U., Held, H., Nakicenovic, N., Scholes, R. J., Talaue-McManus, L., & Turner, B. L. (2004). Abrupt changes: The Achilles' heels of the Earth System. Environment, 46(3), 8-20.