A MONUMENTAL CHALLENGE awaits us later this year in Paris. As time runs out in our battle against dangerous and irreversible climate change, many of us — and here I include myself: a climate scientist best known for the so-called “Hockey Stick” curve — regard the upcoming UN Climate Summit as potentially our last opportunity to achieve the necessary reductions in carbon emissions.

It is against this backdrop, infused with urgency and anxiety, that I read Atmosphere of Hope, the latest contribution from environmental scientist Tim Flannery.

Atmosphere of Hope is a successor to Flannery’s best-selling “climate change”-themed book The Weather Makers. Despite his optimistic-sounding title, Flannery comes across as more sober and less sanguine than he did in Makers, and perhaps even as a bit jaded. Some of his earlier optimism has eroded for obvious reasons: a consequence of a decade and a half of climate policy inaction, and of outright failure to achieve binding cuts in emissions at the 2009 Copenhagen summit.

Yet the book does a remarkably good job of arguing that there is still hope for averting catastrophic climate change, typically defined as planetary warming in excess of 2oC (3.6oF) relative to pre-industrial time. Earth has thus far warmed about 1oC, and another 0.5oC is already in the pipeline. To be sure, the wiggle room is small.

In fact, some pundits have already written off the task as impossible. In the journal Nature, Oliver Geden, head of the EU Research Division at the German Institute for International and Security Affairs, recently asserted that “the climate policy mantra — that time is running out for 2°C but we can still make it if we act now — is a scientific nonsense.” It is an odd and self-defeating claim, given that there is no physical obstacle to 2oC stabilization. Lack of political and collective societal will constitutes the only true obstacle to such stabilization. Insisting that the goal is no longer possible is dangerous insofar as it becomes a self-fulfilling prophecy, providing an excuse for politicians unwilling to support the dramatic actions needed.

It is in confronting this softer, gentler form of denialism — the denial of hope — that Atmosphere of Hope shines. Flannery is no Pollyanna — he fully acknowledges the steep challenges and serious obstacles we face. So when he affirms that a path to averting catastrophic climate change remains in place, we know the conclusion is not reached capriciously. In excruciating detail, in chapter after chapter, he explains why averting catastrophe is indeed still possible.

Flannery closely examines the relevant developments in renewable energy (wind, solar, geothermal); he describes what is already being accomplished, and how these technologies might be scaled up in the years ahead. Most readers will be surprised to learn that renewables are already providing 22 percent of global power generation. Countries like Germany are getting more than 30 percent of their power from renewables. The US, at 15 percent, has fallen behind the rest of the world. But even here there is reason for cautious optimism. The West Coast states and the Northeastern states — home to nearly a third of our population in total — are forming coalitions to tackle the problem. The Obama Administration’s new Clean Power Plan promises progress at the national scale by phasing out climate-unfriendly coal and replacing it with climate-friendly renewable energy.

The president’s critics are fond of branding this new policy a “war on coal.” But the fact is that coal, the most carbon-intensive and climate-unfriendly source of power, is, as Flannery notes, already on the decline worldwide. Can that be a consequence of President Obama’s putative “war on coal”? No, in reality, there is no more a “war on coal” now than there was a “war on whale oil” in the mid-19th century, when it was forced off the market by petroleum. Technologies become antiquated, replaced by something more efficient, and hopefully “better.” As a colleague of mine is fond of saying, “the Stone Age didn’t end for want of stones.” And the fossil fuel age is ending not for want of fossil fuels. We have five times as much fossil fuels in proven reserves as would be needed to warm the planet beyond the 2°C limit. It isn’t scarcity, but necessity, that is ending “the age of fossil fuels.”

Flannery explores the power of social movements and the growing role played by the individual citizen in raising awareness about the climate threat. Just a year ago, 300,000 individuals joined by UN Secretary-General Ban Ki-moon, former US vice president Al Gore, New York City Mayor Bill de Blasio, and other luminaries marched through the streets of New York City in the largest climate change demonstration in history. Similar marches took place in cities around the world.

Following the New York march, the Rockefeller Brothers Fund, representing the estate of John D. Rockefeller — who founded the oil empire that went on to become ExxonMobil — announced that it would be divesting itself of all fossil fuel holdings. That monumental development was part of a larger ongoing divestment campaign, spearheaded by climate change activist Bill McKibben, that has led to more than $50 billion worth of funds being withdrawn by more than 300 foundations and institutions around the US and the world. While its primary importance may be symbolic, the divestment campaign is founded on a simple truth: assuming we cannot possibly afford to tap all of the fossil fuels currently on the industry’s balance sheets, then their key assets — the massive fossil fuel reserves in their possession — must perforce ultimately be stranded. These fossil fuel companies are thus not only a bad investment for the planet, but a bad investment for the wary investor.

Concerted action at the city and municipal level is fueling ground-up (as opposed to just top-down) policy progress. The mayors of several of the largest cities in the US — Los Angeles, Houston, and Philadelphia — have all signed on to an aggressive plan to cut emissions; the plan is known as the Mayors’ National Climate Action Agenda.

But the key question is of course whether these developments are sufficient. Alas, perhaps not. Despite making remarkable progress toward a renewable energy-driven global economy, without additional measures we might not get there fast enough to avoid the 2oC warming threshold. Had we acted a decade and a half ago, when Flannery wrote Makers, the task would have been far easier, the transition to renewables far gentler. The irony is not lost on Flannery.

Decades of inaction — the result of a public disinformation campaign funded by fossil fuel interests to dispute the growing climate threat — has made the task far more difficult. As Flannery explains, we might have to resort to other stop-gap measures. One of them falls in the realm of what’s sometimes called geoengineering: the intentional and additional manipulation of the Earth system in an attempt to reduce warming. Many of these schemes, like shooting reflective sulphur particles into the stratosphere to block some of the sunlight, or “fertilizing” the oceans with iron to increase carbon-scavenging algae, could actually make things worse. With geoengineering, the principle of “unintended consequences” reigns supreme. I concur with Flannery that we probably don’t want to go there unless we have no other choice. For the moment, we have other choices.

Flannery speaks of a “third way,” referring to methods that directly remove carbon from the atmosphere and bury it for the long-term. Whether this is truly a third way is somewhat semantic. Seeding the oceans to remove carbon from the atmosphere, for example, is tampering with the Earth system, and I’m uncomfortable calling it anything other than geoengineering. There are less invasive alternatives, like burning biofuels with carbon capture and “open air capture” (e.g., building synthetic “super trees” that are a thousand times as efficient as “natural” trees in removing and burying atmospheric carbon). These methods try to get the carbon genie back into the bottle, fighting the laws of both thermodynamics and economics. As a result, they’re hard, and expensive. Nonetheless, as the cost of climate change damages continues to climb, these options might soon be on the table.

Whether or not one considers these options “geoengineering,” and whether or not we place them in a separate category from more conventional approaches like incentivizing renewable energy, the challenge in the end remains the same. For them to work, we must put a price on carbon, whether through a carbon tax, cap-and-trade, fee-and-dividend, or by other means. In other words, market mechanisms lead us toward a solution only if we internalize the damage from burning carbon by way of a clear price signal.

Flannery’s exploration of the climate change problem is comprehensive. He covers everything from the underlying basic science to the nitty-gritty details of prospective solutions. The book is at its best when laying out the latter. And, while Flannery understandably emphasizes his home country of Australia, he does a good job, too, of touching on projected impacts, politics, and policy advances in North America, Europe, and the rest of the world. If there are weaknesses, they lie in his discussion early on in the book of certain aspects of the basic science.

At times, Flannery conflates types of uncertainty that are fundamentally different in nature. In several instances, for example, he attributes to “complexity” the fact that studies sometimes come to conflicting conclusions about certain climate change impacts. But that makes it sound like the uncertainties are an irreducible consequence of “complexity” (of chaotic behavior, for instance), when often it’s simply that we’re at the forefront of the science, and different reasonable assumptions or approaches lead to different answers. We’re still working toward a more thorough scientific understanding and more confident answers. A good example is the impact of climate change on hurricane behavior.

When Flannery states early in chapter 1 that “no climate model can predict the future—simply because the future is impossible to predict,” I was disappointed not to see more nuance. Strictly speaking, the statement is incorrect. There are at least two primary factors contributing to current uncertainties in future climate projections. One is the uncertain nature of future human decision-making. Arguably, that uncertainty is irreducible. There is no way to determine how the politics of this issue will play out. But the other primary source of uncertainty is physical uncertainty (e.g., the current uncertainty in the precise role of amplifying mechanisms — “feedbacks” — that may increase the amount of future warming). With additional research and observations, that uncertainty can be diminished. So can uncertainties in regional climate projections that are a product of still imperfect representations of features of the climate system (e.g., the El Nino phenomenon).

In discussing climate change impacts on drought, Flannery missed an opportunity to carefully distinguish between drought and low rainfall. The distinction is critical to understanding the current record drought in California, which is a result of both low rainfall and record warmth, the latter having led to increased evaporation from soils and virtually no snowpack, a triple whammy when it comes to freshwater availability.

Finally, I was perplexed by Flannery’s assertion that certain threats like worsening hurricanes and the drying of the Amazon have been “downgraded.” They haven’t. At best, the claim rests on an incomplete reading of the literature. While Flannery is correct that some studies have found that the number of hurricanes globally would decrease, this is highly misleading. Other leading analyses support an increase. But more importantly, when it comes to hurricanes, what is most relevant is what happens to the strongest storms, which do the vast majority of damage. The latest science suggests an increase in the most intense hurricanes. Missing from Flannery’s discussion of Superstorm Sandy is the key role that sea level rise has already played, in this case having increased coastal flooding by 25 square miles and damages of $8 billion.

Flannery cites a 2013 study by the UK Met Office that questions the extent of drying of the Amazon, but ignores a more recent (2014) study in the Proceedings of the National Academy of Sciences, which concludes that “if drying continues across Amazonia, which is predicted by several global climate models, this drying may accelerate global climate change through associated feedbacks in carbon and hydrological cycles.”

To me, Flannery’s concessions feel like an attempt to demonstrate balance by conceding that certain past predictions were exaggerated. The problem is that a more representative assessment of the literature indicates, at most, current uncertainty in those predictions. But uncertainty is not a reason for comfort. Hindsight makes this especially apparent: Arctic sea ice is decreasing faster than models had originally predicted, and the Antarctic and Greenland Ice Sheet are losing ice and contributing to sea level rise faster than expected.

In the end, however, these (and other nitpicks) were only a minor annoyance for me. There are other places readers can turn for an in-depth and up-to-date discussion of the science. What Flannery provides — a convincing defense for the position that a path to averting catastrophic climate change still exists — is invaluable.

With the world’s two largest emitters of carbon — the US and China — having engaged in a historic agreement to make substantial reductions in their own carbon emissions, the stage is now set. Will the rest of the nations of the world join in and help us reach the binding targets in Paris necessary to avert catastrophe? Will we tackle this problem in time?

Flannery is cautiously optimistic. As am I.

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Michael Mann is Distinguished Professor of Meteorology at Penn State University and was recognized, with other authors, for contributing to the International Panel on Climate Change’s 2007 Nobel Peace Prize. His most recent book, co-authored with Lee Kump, is Dire Predictions: Understanding Climate Change (Pearson/DK publishing, 2015).