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but generally regional and highly diachronous, influence on the Earth System thousands of years ago. With the onset of the Industrial Revolution, humankind became a more pronounced geological factor, but in our present view it was from the mid-20th century that the worldwide impact of the accelerating Industrial Revolution became both global and near-synchronous.¹³

      They rejected the Early Anthropocene proposals because they only address one aspect of the case for a new epoch, human impact on terrestrial ecosystems. “The significance of the Anthropocene lies not so much in seeing within it the ‘first traces of our species’ (i.e., an anthropocentric perspective upon geology), but in the scale, significance, and longevity of change (that happens to be currently human-driven) to the Earth System.”¹⁴

      In January 2016, the AWG majority published a particularly strong statement on whether changes to the Earth System have been sufficient to justify declaring a new epoch, and if so, when the new epoch began. The title of their paper, published in Science magazine and signed by twenty-four AWG members, is unequivocal: “The Anthropocene is functionally and stratigraphically distinct from the Holocene.”

      Interviewed by The Guardian, Colin Waters, lead author of the paper, described the global shift as “a step-change from one world to another that justifies being called an epoch. What this paper does is to say the changes are as big as those that happened at the end of the last ice age. This is a big deal.”¹⁵

      The January 2016 paper summarized recent research that identifies major ways in which Holocene conditions no longer exist:

      • Atmospheric concentrations of CO₂ have exceeded Holocene levels since at least 1850, and from 1999 to 2010 they rose about 100 times faster than during the increase that ended the last ice age. Methane concentrations have risen further and faster.

      • For thousands of years global average temperatures were slowly falling, a result of small cyclical changes in the Earth’s orbit. Since 1800, increased greenhouse gases have overwhelmed the orbital climate cycle, causing the planet to warm abnormally rapidly.

      • Between 1906 and 2005, the average global temperature increased by up to 0.9°C, and over the past 50 years the rate of change doubled.

      • Average global sea levels began rising above Holocene levels between 1905 and 1945. They are now at their highest in about 115,000 years, and the rate of increasing is accelerating.

      • Species extinction rates are far above normal. If current trends of habitat loss and overexploitation continue, 75 percent of species could die out in the next few centuries. This would be Earth’s sixth mass extinction event, equivalent to the extinction of the dinosaurs, 65 million years ago.

      A particularly frightening observation: even if emission levels are reduced, by 2070 Earth will be the hottest it has been in 125,000 years, which means it will be “hotter than it has been for most, if not all, of the time since modern humans emerged as a species 200,000 years ago.”

      Much of the paper focused on a key question for geologists: Has human activity produced a stratigraphic signature in sediments and ice that is distinct from the Holocene? It turns out, contrary to the doubts some expressed at the beginning of this process, that future geologists will have a wealth of indicators to choose from:

      Recent Anthropogenic deposits contain new minerals and rock types, reflecting rapid global dissemination of novel materials including elemental aluminum, concrete, and plastics that form abundant, rapidly evolving “technofossils.” Fossil fuel combustion has disseminated black carbon, inorganic ash spheres, and spherical carbonaceous particles worldwide, with a near-synchronous global increase around 1950.

      Anthropocene ice and sediments are also marked by unique concentrations of chemicals, such as lead from gasoline, nitrogen and phosphorus from fertilizers, and carbon dioxide from burning fossil fuels. But “potentially the most widespread and globally synchronous anthropogenic signal is the fallout from nuclear weapons testing.” Residues from hydrogen bomb explosions that began in 1952 peaked in 1961–62, leaving a clear worldwide signature.

      Each of these stratigraphic signatures is either entirely new or outside of the Holocene range of variability—and the changes are accelerating. The paper recommended that the International Commission on Stratigraphy accept the Anthropocene as a new epoch.

      On the question of when the Anthropocene began, the authors’ analysis was “more consistent with a beginning in the mid-20th century” than with earlier proposed dates. They did not make a specific midcentury recommendation beyond noting that a number of options have been suggested, ranging from 1945 to 1964.

      Finally, they left open the question of “whether it is helpful to formalize the Anthropocene or better to leave it as an informal, albeit solidly founded, geological time term, as the Precambrian and Tertiary currently are.”

      This is a complex question, in part because, quite unlike other subdivisions of geological time, the implications of formalizing the Anthropocene reach well beyond the geological community. Not only would this represent the first instance of a new epoch having been witnessed firsthand by advanced human societies, it would be one stemming from the consequences of their own doing.

      It is still possible that the usually conservative International Commission on Stratigraphy will either reject, or decide to defer, any decision on adding the Anthropocene to the geological time scale, but as the AWG majority writes, “The Anthropocene already has a robust geological basis, is in widespread use, and indeed is becoming a central, integrating concept in the consideration of global change.”

      In other words, failure to win a formal vote will not make the Anthropocene go away.

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      Tipping Points, Climate Chaos, and Planetary Boundaries

      The Anthropocene raises a new question: What are the non-negotiable planetary preconditions that humanity needs to respect in order to avoid the risk of deleterious or even catastrophic environmental change at continental to global scales?

      —JOHAN ROCKSTRÖM¹

      After listing recent critical changes to the Earth System, the authors of Global Change and the Earth System insisted that such lists do not give the whole picture: “Listing the broad suite of biophysical and socioeconomic changes that is taking place fails to capture the complexity and connectivity of global change since the many linkages and interactions among the individual changes are not included.” The listed crises, and others, reinforce and transform one another, producing complex “syndromes of change,” and “many changes do not occur in a linear fashion, but rather, thresholds are passed and rapid, non-linear changes ensue.²

      That understanding of ecological volatility, a recent development in Earth System science, is a direct result of IGBP projects conducted around the world in the 1990s.

       The Past as Guide to the Future

      From the early 1990s, the International Geosphere-Biosphere Program organized its work into nine projects that focused on broad aspects of the Earth System, including terrestrial ecosystems, atmospheric chemistry, and ocean ecosystems. Each project included a multitude of specific studies conducted by scientists around the world.

      All the projects contributed to IGBP’s goal of producing an integrated picture of the nature and direction of global change, but arguably the most important, in both objectives and results, was the Past Global Changes (PAGES) project, charged with “providing a quantitative understanding of the Earth’s past climate and environment.”³ The importance of this work can be stated simply: we cannot understand the dynamics and direction of today’s changing Earth unless we know how current conditions differ from those of the past:

      Understanding the expression, causes, and consequences of past natural variability is of vital concern for developing realistic scenarios of the future. Moreover, the complex interactions between external forcings and internal system dynamics on all timescales implies that at any point in time, the state of the Earth System reflects not only characteristics

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