When do you think we will reach 1/10th of our current GHG emissions?

2020? 2030? 2040? 2050? 2100? Plus or minus five years? Will we have a linear decline to that level? When do you think that decline will start? Is there a particular model you're thinking of?

What degree of GHG concentrations/temperature increase will be the limit of what you consider to be 'not dire for humanity'? Is it compatible with your expected model of GHG emissions?

Are the billion people living in the Indian subcontinent included in your definition of humanity, or are we talking about North Americans and Europeans? If yes, will we be willing to shelter environmental refugees from impacted areas?

I'd really love to believe that everything will turn out fine - but while I've ran into many optimists, I've yet to run into one that is willing to quantify the reasons for their optimism.

My guess is ~2070-2090 will see 20% of current fossil fuel use.

Boats and Aircraft are going to be hardest to transition and will likely make up a large chunk of that 10+%. Bio fuels may take some of this, but it really depends on overall demand for liquid hydrocarbons.

Electricity and home heating are going to change from market forces.

Cars have many options, but oil is going to get expensive (as in 2-3x current price not 10x) which will push alternatives.

Electric trains can take over most long distance trucking and shorter distance is easier to transition. This will be a fairly fast transition if oil spikes again in ~2030-2040 combined with some form of carbon tax.

PS: Plastics / fertilizer and other chemical processes are going to be the wildcard IMO. But, they don't involve burning carbon so they are also less important.

I was writing up a response to your initial guess of 10% by 2060, which was... Pretty dire. Past-the-Paris agreement, 2.5C to 3C warming dire.

20% by 2070-2090, on the other hand, is a lost cause. That would put atmospheric at 580 PPM by 2080. That's >4C of warming, if we don't hit any positive feedback loops.

I'm not particularly concerned about plastics. They can be synthesized from biomass.

Sorry, I got into a habit of submit then edit on HN because we used to get errors if you waited to long. I did not think we would hit 10% by 2060, but I hope we hit 40%.

US coal use is down to where it was around 1930. Petroleum is around 1960's usage levels. https://www.eia.gov/todayinenergy/detail.php?id=26912

Natrual gas has spiked, but it's realitvly speaking less of a global warming threat due to C(x) H(x) turning into H20 (x) + C02 (x) which produces less CO2 per heating vs just C into CO2 from coal.

So, while there are a huge range of predictions. What happens in the next 20 years is more important than what's going on 40 - 60 years from now.

Don't the inevitable methane leaks from natural gas create more of a GHG effect than the CO2 emitted from burning it? It's not just the environment disasters like in California, but everyday minor leaks.

Methane has a short half life in the atmosphere ~10 years ish and there is already quite a bit of it there. So, it's only really important when the amount released is increasing relative to the average released over the last 20 year. AKA if they cut release in 20 years then (ed: it's contribution to) temperature would drop fairly quickly. https://en.wikipedia.org/wiki/Atmospheric_methane

Note it does break down into more CO2 than the wieght of Methane because Oxygen molecules are heavy.

The real risk is if warming the arctic starts to release a lot of stored methane, which would be very bad.

They are not insignificant (right now, they're just a bit more than the methane from livestock) but the CO2 is emitted in much greater amounts.

In a few years the question will become "How to do geoengineering safely relative to the risks of warming?" The planet may be able to support 9-11 billion people with mostly renewable energy, but perhaps humanity will not get there fast enough to avert risk of hitting a positive feedback level that would commit humanity to far more massive and therefore far riskier geoengineering.

The planet is unable to sustainably support the current 7 billion people. The only way I see 9 billion fitting in here is if we switch to purely artificial stuff and eradicate all wildlife and their habitat.

The way I see it, the limit is at 1 billion people if we want to preserve current situation, and even less if we want to let the nature regenerate.

I don't dismiss the idea that we are past sustainable limits already. But the fact is that increased living standards have not caused commodities crunches yet. Also, stabilizing population is the first step toward a sustainable population level, no matter what the right number is for a world that thinks in terms of a "long now."

As far as planes go, an intriguing option is liquid hydrogen. Hydrogen has a large energy density by mass, which is why we use it in rockets. There have been proposals on how current airliners might be converted to run off of hydrogen in the near term[0] for short and medium distance travel. If we can get fuel cell electric propulsion to work at jetliner speeds there is even more potential, as fuel cells can be made more efficient than heat engines.

[0]http://www.fzt.haw-hamburg.de/pers/Scholz/dglr/hh/text_2004_...

I think it would be much easier to use methane. Just as carbon neutral if you make it yourself, and doesn't have as many exotic engineering requirements.

I didn't know that about fuel cells though, interesting.

10% -> 2025; 20% -> 2030; 50% -> 2040; 95% -> 2050. India/Africa are included there. I think 95% of potential GHG emmissions (not just indexed to today but to 'present time') will be crossed in like 2060 or so. Renewables are going extremely quickly. we are in the steep part of the s-curve and there is no reason to believe we wont' get a few doublings due to battery cost decreases and deployed solar cost efficiencies. The transition will be a lot faster than ppl believe.

By that plan, we'd be looking at ~21.6 years of current GHG emissions.

This will add ~777 GT of CO2 to the atmosphere. Which will put us at ~500 ppm CO2 (We just passed 400 ppm). That would be warming between 2.5C and 3C.

If you want to hit a 95% reduction in GHG emissions, you're also going to have to:

* Shut down every single fossil fuel power plant over the next 25 years.

* Build enough renewable/nuclear powerplants to replace all of our fossil fuel power sources, twice over. (Assuming we switch to electrical transportation.)

And, the elephant in the room:

* Halve all current trans-continental shipping and air travel. It is currently responsible for ~6-7% of our GHG emissions. (And if it were to grow unconstrained, would likely double in volume in the next 30 years.)

* While bringing our other emissions down to nearly zero.

We can clearly do all this, but it would require significant changes in our lifestyles - something that optimists tend to not be willing to accept.

Agreed. It is more dire than most scientists are willing to admit or the public are willing to hear. At this point I think the best hope is:

a) incredibly rapid adoption of renewables

b) a Manhattan project to find a way to pull carbon out of the atmosphere starting within ~1-3 decades, once it becomes beyond obvious we've already emitted far too much and the consequences of inaction are more than human civilization can bear

This is the craziest thing I've written in 2017 but: I sincerely believe we will have consumer fusion reactors by 2030 if not sooner. The work of MIT SPARC combined with graphene manufacture will make this a reality. Combine this with all the investment in battery load leveling and electric vehicles and you'll see a very rapid energy cost reduction and deployment. Probably 10 years once the design is functioning. As to the boats, these reactors are well sized for boats. My guess is ICE Air-travel will be completely eliminated for mass consumption by 2070 if not 2050, replaced by faster cheaper evacuated trains and electric short-range aircraft. The power density of li-ion batteries at roughly 350WHr/liter is not great, I'd wager we can easily get to 700WHr/liter with graphene supercapacitors utilizing a combination of 3D layering and fractal interaction. I've seen an experimental graphene supercap with fractal interaction at 1200Whr/Liter but it had a tendency to explode with only a few cycles. The normal graphene supercaps go like 100k cycles with no degradation, or 30 times longer than lithium ion cobalt (what tesla uses). We are so freaking close to cheap graphene it's ridiculous. That's the technology no one sees coming, it's just about to enter the productivity plateau. /end crazy person talk.

Graphene is cheap - you can make it with sticky tape.

Electricity and transportation, which you often see in the headlines about progress about moving to renewables, are not the only GHG emitters. Agriculture, deforestation, heating are also big sources. As far as I have read there has been a lot less progress in that area.

In the US, electricity + transportation makes up 56% of anthropogenic emissions and agriculture makes up 9%:

https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emis...

The European Union reports aggregates a bit differently, but agriculture is only 10% there too (see Figure 3):

http://ec.europa.eu/eurostat/statistics-explained/index.php/...

Neither the USA nor the EU are undergoing deforestation at present, and electrical heating can substitute for the vast majority of applications currently served by fossil-combustion heat. That's why most research and headlines about GHG reductions in the developed world focus on transport electrification and electricity decarbonization.

In poorer parts of the world, where automobile use is less common and/or fewer people have grid tied electrical service, deforestation and agricultural emissions make up a larger share of total anthropogenic emissions. But most of the world's anthropogenic emissions, and most of the low-hanging mitigations for emissions, come from geographic regions that commonly have grid tied electrical service and other modern amenities.

But those replacements, electrical heating and industrial processes, are also holes. Basically, if we replaced 100% of the current electricity production with renewables that wouldn't even provide the energy needed for electric vehicles. So we already have to go >100%. And then we need to overhaul heating and energy-intensive industries too.

And even if we get to that point we still would not be carbon-neutral due to the remaining sectors. And possibly due to new natural sources (permafrost) we're already locked into due to the warming that is already in progress.

So the point is that just inventing our way to replace the current grid with green sources is insufficient for meeting the climate budget within a few decades.

I think we're all making slightly different points. OP said "We will end our reliance on fossil fuels ... by developing green energy tech that is simply more efficient and cost effective than fossil fuels. Once this happens the transition away from carbon based energy sources will be swift."

You replied pointing out that electricity and transportation are not the only sources of emissions -- not sure if it was intended as a rebuttal because OP didn't say they were. I noted that electrical heating can generally substitute fossil heating, that the developed world isn't undergoing deforestation, and that agriculture is a much smaller source of developed-world emissions compared with fossil combustion. Neither I nor OP said or implied that "just inventing our way to replace the current grid with green sources is sufficient for meeting the climate budget."

We're obviously going to blow past the "safe" 2-degree-rise-by-end-of-century cumulative emissions target. In the long term, if complex civilizations survive, I expect large scale atmospheric carbon removal efforts (enhanced silicate weathering and similar). Otherwise feedback processes and anthropogenic emissions apart from fossil combustion are probably going to amplify ~4 degrees of end-of-century warming to more like 7-9 degrees of warming by end-of-millennium. After that it'd take ~100k years to restore the pre-industrial atmospheric CO2 concentration status quo by natural sinking mechanisms alone.

I like your optimism, but I think you're wrong. It may already be too late.

This will make a very nice ecological pressure on humans.

Climactic conditions will always be a problem. The next headache will be how to deal with a heat bath from uniformly dissipated waste heat from this energy rather than natural gradients and storage that drive climactic processes. When greedy humans get their hands on 'unlimited energy' that is not a panacea, but a much bigger catastrophe waiting to happen.

If humans really had "unlimited energy", we could air condition the entire world, and send the waste heat to space with large infrared (or even visible light) radiators.

At some point potentially soon if you want really good economic growth you have to decouple economic growth from energy growth. That is a step more fundamental than decoupling energy growth from coal and oil.

Not really. Air conditioning is a heat displacement system, so you'd still have the same amount of heat. Using infrared radiators would just accelerate global warming since green house gases mainly absorb infrared light.

Visible light would probably be a hazard. Would be pretty great for "beam in the sky" special effects though.

I think she was suggesting sending the exchanged heat into space..you know, up.

It's heat, you can't send it anywhere except by thermodynamic processes. You can't beam heat to space. A beam of infrared is not heat, it is high quality free energy.

Let's say we point that beam at the moon. The Moon gets warmer. So in this thermodynamic system Earth-Moon if Moon gets warmer, the Earth gets colder.

Since we're talking about solar power, this is not a huge concern.

I wonder if it's feasible to mitigate the waste heat radiation problem by combining photovoltaics with thermoelectrics?

meh... people have been screaming DHOOOOOOOOOM!!! for generations upon generation...

We'll never make enough food... we'll never have enough energy... we'll never get along...

Things are always bordering on the end of the world yet we survive.

"Survivorship bias or survival bias is the logical error of concentrating on the people or things that made it past some selection process and overlooking those that did not"

Which human species didn't make it past all of the previous catastrophes they predicted?

Those that didn't are largely not around to let us know whether or not anyone was raising the alarm.

https://en.wikipedia.org/wiki/Maya_civilization

https://en.wikipedia.org/wiki/Rapa_Nui_people

Which INTELLIGENT species didn't make it past all of the previous catastrophes they predicted?

...all the ones we never heard of.

Prior returns don't guarantee future results. Have you considered that people have taken steps to avoid catastrophy.

Granted, it only takes one "failure"... but... "people taking steps" is exactly what I was alluding too...

Time and time again, cries of "Too many people!!!"... how will we feed 1 billion people?

Advances in farming, logistics, technology, etc...

What's being done now? More advances in farming, vertical farming, AI, etc...

That depends on the nature of the threat. Running out of feedstock for plastic? Not a problem. That's the sort of thing for which one can assume a technology fix will happen, and there is probably plenty of time. On the other hand, missing the population slow-down projections and ending up with a 20 billion population? That's doom, and there is no techno-savior.

"ending up with 20 Billion Population? That's doom, and their is no techno-savior"

Except for advances in vertical farming, green energy, advances in urban planning allowing people to live denser and more packed together...

We'll all be dead because earth can't handle the population... cira'1968:

https://www.nytimes.com/2015/06/01/us/the-unrealized-horrors...

"Dr. Ehrlich was so sure of himself that he warned in 1970 that “sometime in the next 15 years, the end will come.” By “the end,” he meant “an utter breakdown of the capacity of the planet to support humanity.”

So... by 1985, we will all be dead because DOOM!!!

Time and time again...

Now... can we all "get along"? Christians, Muslims, Chinese, Russians, etc? THAT isn't a technology problem - that's a humanity problem.

Can we feed everyone? We sure can... and advances in tech continue to push the envelope and there is no reason to believe that we can support many multiples of what we have now.

Is there an upper limit? Undoubtedly... are we near it? Hardly...

A population that's 2X the fairly optimistic projections of global population growth slowdown roughly double energy needs, at a time when, at best, renewable energy faces both enormous opportunity and enormous challenges in replacing coal and oil, and when cutting greenhouse gasses is critical to human survival. Just making that twice as large a challenge is reckless and dices with extinction.