The Earth's climate has changed. After nearly two centuries of fossil fuel-burning, concentrations of carbon dioxide in the atmosphere have reached 400 parts per million, especially boosted by the seemingly ever-accelerating amount of combustion in the last few decades according to the World Meteorological Organization. Atmospheric CO2 concentrations of 0.04 percent may not seem like much but it is enough to have already raised average global temperatures by a full degree Celsius, according to the U.K.'s Met Office, with more warming on the way as the greenhouse gas lingers invisibly in the atmosphere, trapping heat, or mixing into the ocean, rendering its waters more acidic.

In fact, the world has not seen CO2 concentrations this high in at least hundreds of thousands of years. Roughly 35 billion metric tons of CO2 are spewed into the atmosphere annually—and rising. The waters of the global ocean have become 30 percent more acidic in the last few decades and the world has not been this warm in thousands of years. This year is likely to be the hottest one since record keeping began, thanks to an El Nino weather pattern that’s taking place in addition to global warming. The top 10 warmest years have all occurred since 1998, which was the year of the last major El Nino.

Worse, farming, forest-clearing and other activities have contributed to emissions of other greenhouse gases, such as methane and nitrous oxide, the latter more commonly known as laughing gas, which is no laughing matter in the atmosphere.

Globally averaged CO 2, CH 4, and N 2 O mole fraction (a) and its growth rate (b) from 1984 to 2014. Differences in successive annual means are shown as shaded columns in (b). Courtesy of WMO

Yet there are signs of hope as well. The U.S. is burning less coal; Europe, and even China have begun to use less of this dirtiest of fossil fuels. And although India and the rest of Asia are building hundreds of coal-fired power plants, there are also plans for more electricity derived from the sun in India, wind in China and hot rocks in Indonesia. In fact, renewables are growing fast all over the world, helping keep more CO2 out of the air. Half of the world's electricity could come from less climate polluting sources by 2040, according to the International Energy Agency's latest World Energy Outlook report. The electric output of renewables alone in 2040 may match the electric output of fossil fuel-fired power plants in China, the European Union and the U.S. today.

Already, China, the European Union, and U.S.—the world's largest polluters, together responsible for more than half of global pollution—have agreed to limit future greenhouse gas emissions. Compared with 1997 when the Kyoto Protocol to combat climate change was agreed on or 2009 when another effort to craft a global deal collapsed in Copenhagen, the prospects for a global effort to combat climate change have never been better. When climate negotiations get under way in Paris later this month, there is a real chance for a comprehensive set of actions from more than 190 nations around the world, all to restrain global warming.

There are yet more signs of hope, like the fact that cities, states, provinces and even nations have started to put a price on CO2 pollution in a bid to cut back its use. We also now live in a world where an infrastructure project like the Keystone XL Pipeline to connect Alberta's tar sands with Texas's heavy oil refineries can be rejected because of its perceived impact on climate change. In other words, what once seemed impossible—stopping a fossil fuel project in a world that derives nearly 90 percent of its energy from fossil fuels—has become not only possible, but reality.

There is still a long way to go, as that 90 percent figure implies. The gap between what nations have promised—reductions of as much as 11 billion metric tons of greenhouse gases by 2030—and what is needed to avoid adding another degree C to the global average temperature—an additional reduction of at least seven billion metric tons of CO2e by 2030—remains large. There are thousands of fossil fuel-fired power plants, more than a billion petroleum-powered vehicles and all the attendant infrastructure to service them, whether oil pipelines, roads or coal mines. We may need technologies like CO2 capture and storage, even if only to deal with the climate-`changing pollution from natural gas-fired power plants or the cement kilns and steel foundries needed to build wind turbines or nuclear reactors.

In fact, we have burned through so much of the world's roughly one trillion metric tons of carbon budget that we will either need a time machine to travel back and change course or methods to draw CO2 back out of the atmosphere, whether through an outbreak of verdure and biochar, phytoplankton blooms and burials, artificial trees or, most likely, all of the above methods—and more.

We have entered what might be called the Anthropocene thermal maximum, an era of global warming driven by one species penchant for burning fossil fuels and cutting down forests. Right now in 2015 may be the last time anyone breathes air with average CO2 concentrations below 400 ppm, as this number marches seemingly inexorably upward. But we don't have to keep adding to that number forever.