What is the biggest change in the new climate report by the world’s top scientists and governments compared to the one they released back in 2007? It can be summed up in one word: “Irreversible.”

In the 2007 assessment of climate science by the U.N. Intergovernmental Panel on Climate Change (IPCC), that word appeared only 4 times in the final, full “synthesis” report. Irreversibility only received 2 mentions and minimal discussion in the Summary for Policymakers (SPM).

Seven years later, the word appears 31 times in the full synthesis report of the IPCC’s fifth assessment. The SPM mentions “irreversible” 14 times and has extended discussions of exactly what it means and why it matters.

Certainly the fact that we are on track to harm billions of people who contributed little or nothing to their harsh fate makes climate inaction a grave “wrong.” But what makes our current inaction uniquely immoral in the history of homo sapiens is that the large-scale harm is irreparable on any timescale that matters — and, of course, that we could avoid the worst of the irreparable harms at an astonishingly low net cost.

What do the world’s leading scientists mean by “irreversible impacts”?

Warming will continue beyond 2100 under all RCP scenarios except RCP2.6. Surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO2 emissions. A large fraction of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial time scale, except in the case of a large net removal of CO2 from the atmosphere over a sustained period….

It is virtually certain that global mean sea-level rise will continue for many centuries beyond 2100, with the amount of rise dependent on future emissions.

Translation: Impacts will be even worse than described in this report after 2100 in every case but the one where we sharply cut CO2 emissions starting now (to stabilize at 2°C total warming). And as high as total warming ultimately gets, that’s roughly as high as temperatures will stay for hundreds of years after we bring total net human-caused carbon pollution emissions to zero.

The “case of a large net removal of CO2 from the atmosphere over a sustained period” means a time far beyond when humanity has merely eliminated total net human-caused emissions — from deforestation and burning fossil fuels (and from whatever amplifying carbon-cycle feedbacks we have caused, such as CO2 and methane release from defrosting permafrost).

To even start reversing the irreversible, we have to go far below zero net emissions to actually sucking vast quantities of diffuse CO2 out of the air and putting it someplace that is also permanent, which we currently do not know how to do at scale at any plausible price. One can envision such a day when we might — if we sharply reduce net carbon pollution to zero by 2100, as we must to stabilize near 2°C. But it’s hard to imagine when it would ever happen if emissions are anywhere near current levels (let alone higher) by 2100, and we have unleashed myriad amplifying carbon cycle feedbacks that make the job of getting to even zero net emissions doubly difficult.

If we don’t get on a very different emissions path ASAP, then some of the most serious climate changes caused by global warming could last 1000 years or more. The SPM explains, “Stabilisation of global average surface temperature does not imply stabilisation for all aspects of the climate system.” That is to say, if we don’t quickly embrace the 2C emissions path, then even at a point many hundreds of years from now when temperatures start to drop, some changes in the climate — sea level rise being the most obvious example — will likely keep going and going.

The IPCC reports are primarily reviews of the scientific literature, so the new focus on the irreversible nature of climate change is no surprise. In 2009 we reported on research led by NOAA scientists titled “Irreversible climate change because of carbon dioxide emissions,” which concluded “the climate change that is taking place because of increases in carbon dioxide concentration is largely irreversible for 1,000 years after emissions stop.”

Significantly, that NOAA-led study warned that it wasn’t just sea level rise that would be irreversible:

Among illustrative irreversible impacts that should be expected if atmospheric carbon dioxide concentrations increase from current levels near 385 parts per million by volume (ppmv) to a peak of 450-600 ppmv over the coming century are irreversible dry-season rainfall reductions in several regions comparable to those of the ”dust bowl” era and inexorable sea level rise.

Recent studies strongly support that finding for both sea level rise and Dust-Bowlification of some of the world’s most productive agricultural lands.

Significantly, this 2014 Synthesis report is the first time I have seen the world’s leading scientists and governments explain why the irreversibility of impacts makes inaction so uniquely immoral. Here is the key finding (emphasis in original):

Without additional mitigation efforts beyond those in place today, and even with adaptation, warming by the end of the 21st century will lead to high to very high risk of severe, widespread, and irreversible impacts globally (high confidence). Mitigation involves some level of co-benefits and of risks due to adverse side-effects, but these risks do not involve the same possibility of severe, widespread, and irreversible impacts as risks from climate change, increasing the benefits from near-term mitigation efforts.

That is a tremendously important argument. Sure, the climate panel says, mitigation efforts have risks in addition to their co-benefits — “possible adverse side effects of large-scale deployment of low-carbon technology options and economic costs,” as the full report explains. But the risks involved in reducing greenhouse gas emissions are both quantitatively and qualitatively different than the risks stemming from inaction because they aren’t likely to be anywhere near as “severe, widespread, and irreversible.”

The full report expands on this critical point, noting that “Climate change risks may persist for millennia and can involve very high risk of severe impacts and the presence of significant irreversibilities combined with limited adaptive capacity.” In sharp contrast, “the stringency of climate policies can be adjusted much more quickly in response to observed consequences and costs and create lower risks of irreversible consequences.”

In short, if some component of the mitigation strategy turns out to start having unexpected, significant negative consequences, humanity can quickly adjust to minimize costs and risks. But inaction — failing to embrace aggressive mitigation — will lead to expected climate impacts that are not merely very long-lasting and irreversible, but potentially beyond adaptation.

Finally, yes, I realize that if humanity is not motivated by the genuine prospect of ruining a livable climate for our children and grandchildren, then how much we screw future generations after 2100 isn’t going to move people. But you can’t blame scientists for thinking homo sapiens is actually a rational and moral species, capable of caring about people who haven’t even been born yet.

The founding fathers certainly cared about such future generations and understood how obviously immoral it was to subject them to irreversible adverse impacts. As The Constitutional Law Foundation has explained, “The most succinct, systematic treatment of intergenerational principles left to us by the founders is that which was provided by Thomas Jefferson in his famous September 6, 1789 letter to James Madison.”

I summarized Jefferson’s position here. The key question for Jefferson was very simple: Must later generations “consider the preceding generation as having had a right to eat up the whole soil of their country, in the course of a life?” Soil was an obvious focal point for examining the issue of intergenerational equity for a Virginia planter like Jefferson.

The answer to Jefferson was another self-evident truth:

Every one will say no; that the soil is the gift of God to the living, as much as it had been to the deceased generation….

It is immoral for one generation to destroy another generation’s vital soil — or its livable climate. Hence it is unimaginably immoral to Dustbowlify their soil and ruin their livable climate irreversibly for many centuries if not millennia. So let’s not do that, okay?