Erle Ellis, who authored "Planet of No Return" for Issue 2 of the Breakthrough Journal, replies here to responses from Bill McKibben, Nils Gilman, Robert Dello-Russo, Ronnie Hawkins and Francisco Seijo.

My goal with "Planet of No Return" was to explain the emergence of the Anthropocene and its implications for the future of humanity.For a complete treatment, see: Ellis, E. C. 2011. "Anthropogenic transformation of the terrestrial biosphere." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Science 369:1010-1035.It seems that the brevity and provocative nature of my essay have managed to inspire remarkably diverse criticisms.

Ronnie Hawkins likens my thinking to that of "a sentient bacterial culture confidently asserting" that "perpetual growth" is possible "while sucking dry its petri dish." In transferring this textbook biological metaphor (the inevitable collapse of exponentially growing bacteria populations) to the dynamics of human systems, we see a perfect example of the failures of old-school environmental thinking.For a classic example of old-school environmental thinking, see: Ehrlich, P. 1968. The Population Bomb. Ballantine Books, New York.

It is not "glib," "cheap," or "cheerful" (McKibben) to point out that the old-school environmental model fails to describe the dynamics of human systems over the long term. The classic biological paradigm simply cannot explain how human populations were able to reach one billion by 1800 and how they will almost certainly reach nine billion by 2050. Human populations have been sustained far beyond their "natural limits" for millennia -- not by benevolent nature, but by increasingly engineered environments and increasing use of energy. It is not Earth's ecology, but our humanity -- in the form of human systemsEllis, E. C., and P. K. Haff. 2009. "Earth Science in the Anthropocene: New Epoch, New Paradigm, New Responsibilities." EOS Transactions 90:473-- that has enabled Homo sapiens to emerge as the single most powerful species on this planet. The transformative power of human systems has grown and developed over the long term as the cumulative product of increasingly larger scales of social and environmental interaction and experience, innovation and learning and the accumulation of social and material capitals. It is this long-term process of human system growth and development that has changed the game for Homo sapiens and the entire planet.

My essay did not state that human population growth and societal development will be unlimited and continuous, will never experience collapse, or that social systems or other technologies will always enable humanity to overcome all biophysical limits. Indeed, the dynastic nature of human history argues that periodic social disintegration and re-emergence built on the gains and ruins of the past may be an inevitable dynamic of human systems.This idea has been termed a social "adaptive cycle." For more, see: Holling, C. S. 2001. "Understanding the Complexity of Economic, Ecological, and Social Systems." Ecosystems 4:390-405.Nevertheless, the claim that human civilizations facing environmental limits or rapid environmental change must inevitably collapse simply fails the test of time.For a rebuttal to this idea of inevitable collapse, see an excellent new special feature: Butzer, K. W., and G. H. Endfield. 2012. "Critical perspectives on historical collapse." Proceedings of the National Academy of Sciences 109:3628-3631.

As I argued, it is time to shift our focus from hard biophysical limits and insurmountable environmental stressors to the resilience and adaptability of human systems.DeFries, R., E. Ellis, F. S. Chapin III, P. Matson, B. L. Turner II, Arun Agrawal, P. Crutzen, C. Field, P. Gleick, P. Kareiva, E. Lambin, E. Ostrom, P. Sanchez, J. Syvitski, and D. Liverman. 2012. "Planetary Opportunities: A Social Contract for Global Change Science to Contribute to a Sustainable Future." BioScience 6: in press.This is not a new idea: ancient scholars determined that famines associated with sustained droughts and the fall of dynasties were caused primarily by social failures -- corruption and decline in the management of taxes and granaries. I couldn't agree more with Francisco Seijo that it is the state of our human systems that determines our future even more than our powers to transform nature. Social power can overcome environmental limits, and this power underpins both the present and future state of humanity and the planet.

Understanding the dynamics of the past and the present cannot entirely predict the future -- to answer all of the "questions about our particular, and pivotal, time" (McKibben). Global climate change caused by fossil fuel combustion will almost certainly diminish potential agricultural productivity in many regions while simultaneously increasing productivity in others.Battisti, D. S., and R. L. Naylor. 2009. "Historical Warnings of Future Food Insecurity with Unprecedented Seasonal Heat." Science 323:240-244.Climate change is indeed an unprecedented challenge to human systems, together with the increasing demands of our larger and wealthier populations. Yet humanity has never been better prepared to respond to this major global restructuring of agricultural opportunities. Just as famines are not caused by drought but by social failures,Sen, A. 1989. "Food and Freedom." World Development 17:769-781.recent dynamics in weather are not the cause of changes in food security. Instead, access to food will continue to be determined by social systems. As in the past, there will be winners and losers, and the more powerful human systems will tend to adapt and thrive, while weaker systems will struggle. This is not a new problem, but an old one: How can we continue to improve the availability of food and other resources for all of humanity?

Finally, a word about my "positive" message about the Anthropocene. As I noted, there is nothing particularly good about a much warmer world (or the loss of wild fish -- my apologies for not being clearer -- the loss of wild freshwater fish as part of our diet.More than 75% of Freshwater fish in the food supply now come from aquaculture. See: Food and Agriculture Organization of the United Nations. 2010. Fisheries and Aquaculture Department Statistics.) That human systems tend to thrive in proportion to their ability to transform and use ecosystems is not to say that altering ecosystems is good or that altered ecosystems are better. That Paleolithic humans may be happier or better off than Agricultural or Industrial humans (Dello-Russo) has no bearing on whether some of these human systems are more capable of sustaining larger, wealthier and more powerful populations that tend to outcompete the others when resources become limited. Nor is it wise for us to test the hypothesis that anthropogenic climate change will make the planet less "hospitable to human life" (Hawkins). Yet I am convinced that human systems have the ability to adapt to this change, and perhaps even avoid it by transforming our energy systems away from fossil fuels (Gilman), a process that will inevitably take half a century or longer, no matter how fast we want it to come.Smil, V. 2010. Energy transitions: History, Requirements, Prospects. Praeger Publishers.This does not mean that I am advocating "the increasingly centralized, fossil-fuel subsidized, engineered road" (Hawkins), but merely recognize that this is the current state of human systems and the planet, and that there is no immediate alternative to support nine billion and more on Earth.

Even with recent dramatic acceleration in our numbers, energy use and transformation of the Earth system, including current and future changes in climate, the history of human systems is excellent evidence of their adaptability and resilience in the face of environmental challenges. It is essential to recognize that even as we increasingly transform the planet, not all of these changes are for the worst, and some processes, like rapid urbanization, agricultural intensification and globalization are potentially good news. It is time to accept that our challenge is not to avoid environmental catastrophe, but to improve our ability to guide human systems towards better outcomes for ourselves, our progeny, and our ecological heritage.