What would it take to really tackle climate change? No delays, no gimmicks, no loopholes, no shirking of responsibility — the real thing. What would it look like?

To answer that question, it helps to understand the upper threshold of climate ambition. The target agreed upon by the world’s nations in Paris in 2015 is global warming of “well below” 2 degrees Celsius, with good-faith efforts to hold temperature rise to 1.5 degrees.

Countries are not moving anywhere near fast enough to hit those targets, so we are currently on track for somewhere around 3 degrees. It is generally agreed that hitting 2 degrees would be quite ambitious, while hitting 1.5 would be nothing short of miraculous. Yet the scientists at the Intergovernmental Panel on Climate Change, in their latest report, are pleading with the world to go for it, because at this point, every fraction of a degree of warming matters.

While there is nothing like a real-world plan in place for hitting those targets yet, climate modelers have come up with many scenarios for how we might do so. However, as I wrote recently, most of those scenarios rely heavily on “negative emissions” — ways of pulling carbon dioxide out of the atmosphere. If negative emissions technologies can be scaled up later in the century, the reasoning goes, it gives us room to emit more earlier in the century.

And that’s what most current 2- or 1.5-degree scenarios show: Global carbon emissions rise in the short term, then plunge rapidly to become net negative around 2060, with gigatons of carbon subsequently captured and buried over the remainder of the century. The oil giant Shell released a scenario along those lines a few weeks ago.

The primary instrument of negative emissions is expected to be BECCS: bioenergy (burning plants to generate electricity) with carbon capture and sequestration. The idea is that plants absorb carbon as they grow; when we burn them, we can capture and bury that carbon. The result is electricity generated as carbon is removed from the cycle — net-negative carbon electricity.

Most current scenarios bank on a lot of BECCS later in the century to make up for the carbon sins of the near past and near future.

One small complication in all this: There is currently no commercial BECCS industry. Neither the BE nor the CCS part has been demonstrated at any serious scale, much less at the scale necessary. (The land area needed to grow all that biomass for BECCS in these models is estimated to be around one to three times the size of India.)

Maybe we could pull off a massive BECCS industry quickly. But banking on negative emissions later in the century is, at the very least, an enormous, fateful gamble. It bets the lives and welfare of millions of future people on an industry that, for all intents and purposes, doesn’t yet exist.

Plenty of people reasonably conclude that’s a bad idea, but alternatives have been difficult to come by. There hasn’t been much scenario-building around truly ambitious goals: to zero out carbon as fast as possible, to hold temperature rise as close to 1.5 degrees as possible, and, most significantly, to do so while minimizing the need for negative emissions. That is the upper end of what’s possible.

In May, I looked at three publications that help fill that gap:

“Global Energy Transformation: A Roadmap to 2050,” by the International Renewable Energy Agency (IRENA), is a plan that targets a 66 percent chance of staying below 2 degrees, primarily through renewable energy.

The analysts at Ecofys recently released a scenario for zeroing out global emissions by 2050, thus limiting temperature to 1.5 degrees and eliminating (most of) the need for negative emissions.

A group of scholars led by Detlef van Vuuren of the Netherlands Environmental Assessment Agency published a paper in Nature Climate Change investigating how to hit the 1.5 degree target while minimizing the need for negative emissions.

Here’s how this post (first published in May) is going to go: First, we’ll have a quick look at why targeting 1.5 degrees is so urgent; second, we’ll look at a few things these scenarios have in common, the baseline for serious ambition; third, we’ll look more closely at the third paper, as it offers some interesting alternatives (like, oh, mass vegetarianism) to typical carbon thinking; and finally, I’ll conclude.

Why targeting 1.5 degrees is urgent

Americans can’t make much sense out of Celsius temperatures, and half a degree of temperature doesn’t sound like much regardless. But the difference between 1.5 and 2 degrees of global warming is a very big deal. (Read the IPCC’s new science review here.)

Another recent paper in Nature Climate Change makes the point vividly: Bumping ambition up from 2 to 1.5 degrees would prevent 150 million premature deaths through 2100, 90 million through reduced exposure to particulates, 60 million due to reduced ozone.

“More than a million premature deaths would be prevented in many metropolitan areas in Asia and Africa,” the researchers write, “and [more than] 200,000 in individual urban areas on every inhabited continent except Australia.”

That’s not nothing! And of course, the difference between 1.5 and 2 degrees could mean the difference between life and death for low-lying islands.

There’s no time to waste. In fact, there may be, uh, negative time. Limiting temperature rise to 1.5 degrees is possible, even in theory, only if the “carbon budget” for that target is at the high end of current estimates.

Again: 1.5 is only possible if we get started, with boosters on, immediately, and we get lucky. Time is not running out — it’s out.

What’s required to limit temperature rise to 1.5 degrees

The three scenarios I mentioned are different in a number of ways. The first two project through 2050, but the Nature Climate Change paper goes out to 2100. They target different things and use different tools. But they share a few big action items — features that any ambitious climate plan will inevitably involve.

1) Radically increase energy efficiency

Just how much energy will be needed through 2050? That depends on population and economic growth, obviously, but it also depends on the energy intensity of the world’s economies — how much primary energy they require to produce a unit of GDP.

Increasing energy efficiency (which, all else being equal, reduces emissions) is in a race with population and economic growth (which, all else being equal, increases them). To radically decarbonize with minimal negative emissions, efficiency will need to outrun growth. (Notably, Shell’s scenario shows much higher global energy demand in coming decades; growth outruns efficiency.)

IRENA’s scenario reduces global energy-related emissions 90 percent by 2050. Of that 90 percent, 40 comes from energy efficiency.

To do this, IRENA says, the energy intensity of the global economy must fall two-thirds by 2050. Improvements in energy intensity will have to accelerate from an average of 1.8 percent a year from 2010 to 2015 to an average of 2.8 percent a year through 2050.

In the Ecofys scenario, energy efficiency is so amped up that total global energy demand is lower in 2050 than today, despite a much larger population and a global economy three times larger than today’s.

The Nature Climate Change paper summarizes the necessary approach to efficiency this way: “Rapid application of the best available technologies for energy and material efficiency in all relevant sectors in all regions.”

“All relevant sectors in all regions” means electricity, transportation, buildings, and industry, all bumped up to the most efficient available materials and technologies, everywhere in the world, starting immediately. Cool, cool, cool.

2) Radically increase renewable energy

All the scenarios envision renewables (primarily wind and solar) rapidly coming to dominate electricity. In the IRENA scenario, renewables grow sixfold faster than they are currently, supplying 85 percent of global electricity by 2050.

Ecofys has them supplying 100 percent of global electricity — with that sector completely decarbonized — by 2040, even as global demand for electricity triples.

The Nature Climate Change paper notes that the vision of rapid renewables dominance all these scenarios have in common involves “optimistic assumptions on the integration of variable renewables and on costs of transmission, distribution and storage,” which, yeah.

3) Electrify everything!

Notably, all three scenarios heavily involve electrification of sectors and applications that currently run on fossil fuels. In the IRENA case, electricity rises from 21 percent of total global energy consumption today to 40 percent by 2050.

In the Ecofys scenario, it rises to a whopping 70 percent. In the Nature Climate Change study, it rises to 46 percent (compared to 31 percent in the reference case).

I have made the case for electrification before, and it’s not complicated. We know how to radically increase the supply of zero-carbon electricity; increasing the supply of zero-carbon liquid fuels is much more difficult. So it makes sense to move as much energy use as possible over to electricity, particularly vehicles, home heating and cooling, and lower-temperature industrial applications.

The Ecofys scenario makes it particularly clear: If renewable energy and energy efficiency are to be your primary decarbonization tools (more on that in a second), full decarbonization requires going all out on electrification.

4) And still maybe do a little negative emissions

Even though the intentions, of the Ecofys and Nature researchers particularly, was to minimize the need for negative emissions, neither was able to completely eliminate it.

“Regardless of the rapid decarbonisation” in the scenario, Ecofys researchers write, “the 1.5°C carbon budget is most likely still exceeded.” The only way to hold at 1.5 is to mop up that excess carbon with negative emissions. Ecofys thinks CCS applications will mostly be confined to industry and the rest can be taken care of by “afforestation, reforestation, and soil carbon sequestration,” i.e., non-CCS methods of negative emissions. And, it notes, this remaining excess carbon “is significantly less than most other low carbon scenarios.”

In the Nature Climate Change study, the need for BECCS can be completely eliminated only if every single one of the other strategies is maximized (see the next section).

Here’s what those researchers conclude about negative emissions:

[W]hile this study shows that alternative options can greatly reduce the volume of CDR [carbon dioxide removal] to achieve the 1.5°C goal, nearly all scenarios still rely on BECCS and/or reforestation (even the hypothetical combination of all alternative options still captured 400 GtCO2 by reforestation). Therefore, investment in the development of CDR options remains an important strategy if the international community intends to implement the Paris target.

They advise policymakers (wisely, it seems to me) to pursue negative emissions strategies but to think of alternative scenarios as insurance against the possibility that those strategies run up against unanticipated social or economic barriers.

Decarbonization beyond renewable electricity and efficiency

The IRENA and Ecofys scenarios, like most rapid decarbonization scenarios, rely overwhelmingly on renewable energy and energy efficiency. But as environmentalist Paul Hawken reminds us with his Drawdown Project, there are more things in heaven and earth than are dreamt of in most climate policy. (For instance, we’re going to talk about fake meat here in a minute.)

Like most climate-economic modelers, the Nature Climate Change researchers use integrated assessment models (IAMs) to generate their scenarios. They tested their decarbonization strategies against the second of five shared socioeconomic pathways (SSPs), which are the modeling community’s set of different visions for the future — different mixes of population, economic growth, oil prices, technology development, etc. SSP2 contains roughly median predictions. (If you’re curious about SSPs, here’s an explainer.)

But they also challenge some of the limitations in how IAMs have typically been used:

As IAMs select technologies on the basis of relative costs, they normally concentrate on reduction measures for which reasonable estimates of future performance and costs can be made. This implies that some possible response strategies receive less attention, as their future performance is more speculative or their introduction would be based on drivers other than cost, such as lifestyle change or more rapid electrification.

The Nature Climate Change paper attempts to model some of these more ambitious, uncertain, or non-cost-driven strategies, assembling a whole suite of decarbonization scenarios in different combinations.

Several of them are familiar: There’s a “uniform carbon tax in all regions and sectors,” along with maximized energy efficiency and renewable energy. But others are more novel in these modeling contexts.

Agricultural intensification: “High agricultural yields and application of intensified animal husbandry globally.”

Low non-CO2: “Implementation of the best available technologies for reducing non-CO2 emissions and full adoption of cultured meat in 2050.” (Non-CO2 greenhouse gases include methane, nitrous oxide, black carbon, fluorocarbons, aerosols, and tropospheric ozone. Cattle are a big source of methane, thus the cultured meat.)

Lifestyle change: “Consumers change their habits towards a lifestyle that leads to lower GHG emissions. This includes a less meat-intensive diet (conforming to health recommendations), less CO2-intensive transport modes (following the current modal split in Japan), less intensive use of heating and cooling (change of 1°C in heating and cooling reference levels) and a reduction in the use of several domestic appliances.” Though they don’t call it out specifically, this would very much involve less flying, one of the most carbon-intensive habits of the affluent.

Low population: “Scenario based on SSP1, projecting low population growth.” Population growth can be curbed most effectively through access to family planning and education of girls (which, notably, have many other benefits as well).

You can decide for yourself how likely you find any of these changes. The researchers say they are modeling “ambitious, but not unrealistic implementation.”

Reducing non-CO2 GHGs and widespread lifestyle changes have the most short-term impact on emissions. However, “by 2100,” they write, “the strongest reductions are found in the renewable electrification and low population scenarios.” This echoes what the Drawdown Project found, which is that educating girls and making family planning widely available (thus reducing population growth) is the most potent long-term climate policy.

Deep thoughts

Needless to say, accomplishing any one of these goals — a global carbon tax, maximized efficiency, an explosion of renewable energy, a wholesale revolution in agriculture, rapid reduction of non-CO2 GHGs, a rapid shift in global lifestyle choices, and successful measures to curb population growth — would be an enormous achievement.

To completely avoid BECCS while still hitting the 1.5 degree target, we would have to accomplish all of them.

That is highly unlikely. Still, the important point of the Nature Climate Change research remains: “alternative pathways exist allowing for more moderate use and postponement of BECCS.” Given the substantial and uncharted difficulties facing BECCS, policymakers owe those alternative pathways a look.

Obviously these strategies face all kinds of social and economic barriers. (I’m trying to envision what it would take to rapidly shift Americans from beef to cultured meat ... trying and failing.) But they also come with co-benefits. Reducing fossil fuels reduces local air pollution and its health impacts. Energy efficiency reduces energy bills. Eating less meat and driving less are healthy.

Overall, a radical energy transition would mean a net boost in global GDP (relative to the reference case) in every year through 2050.

An energy transition would also create millions of net jobs. But that doesn’t mean it will be easy.

Engineering any of these shifts, the Nature Climate Change researchers write with some understatement, “requires not only insights from IAMs, but also in-depth knowledge of social transitions.” They suggest (and I heartily endorse) that subsequent research focus on social and political barriers and strategies.

In the end, perhaps the most important conclusion in the Nature Climate Change paper is the simplest and the one that we already knew: “a rapid transformation in energy consumption and land use is needed in all scenarios.”

At this point, whether it’s possible to hit various targets is almost beside the point. All the science and modeling are saying the same thing, which is that humanity faces serious danger and needs to reduce carbon emissions to zero as quickly as possible.

The chances of us getting our collective shit together and accomplishing what these scenarios describe are ... slim. There are so many vested interests and so much public aversion to rapid change, so many governments to be coordinated, so many economic and technology trends that must fall just the right way. It’s daunting.

Conversely, the chances of us overdoing it — trying too hard, spending too much money, reducing emissions too much or too fast — are effectively nil.

So the only rule of climate policy that really matters is: go as hard and fast as possible, forever and ever, amen.