and the supercritical CO2 heat pumps used in Japan are used for hot water because chlorofluorcarbons are not efficient for high temperatures.

"In Europe, heat pumps enjoyed a record year, with sales growing by nearly 40%. In particular, sales of air-to-water models, which are compatible with typical radiators and underfloor heating systems, jumped by almost 50% in Europe. In the United States, heat pump purchases exceeded those of gas furnaces."

Not sure what reason is for the apparent contradiction, but the planned additions to US electricity generation seem to be mostly solar.

Perhaps, but even with a pure free market, and not imposing the external costs of coal (like health and climate change) on the industry, I can't see how the US electricity generation industry will continue using coal plants beyond the working life of the existing plants. Solar is just so cheap, and under free market theories it will get cheaper as volume manufacture of the panels increases. It seems to me therefore that the MIT graph of coal use out to 2100 cannot possibly be correct whether or not we like free markets.

We might reasonably argue that US electricity generation using coal plants is not representative of the global coal industry, but 92% of India's new electricity generation capacity was solar in 2022 and although they are building lots of coal plants China seems to be building solar at approximately the same rate as coal. The IEA says global coal increased 1.6% in 2022, mostly in Asia, but they also say renewables met 90% of last year’s global growth in electricity generation. So I guess we are at approximately global peak coal. After peak coal the MIT graph should go downwards. Exactly when and how fast it goes down is open to debate, for example the UK shut down 90% of its coal in 10 years, but MIT have drawn the graph going upwards to 2100. That's why I think the MIT graph is plain wrong.

The nuclear crowd are likely just part of the disinformation campaign: Investigating climate sceptics’ disinformation strategy on Twitter, by Sebastián Escalón

The MIT graphs look plain wrong. For example, the left-hand graph for heavily subsidize renewables shows coal, oil and gas as primary energy sources effectively flat or slightly increasing out to 2100. The current growth of solar and wind is already reducing fossil fuels in major areas, for example in the US for electricity generation, this will accelerate as solar becomes cheaper.

List of all figures in AR6:

• There are multiple opportunities for scaling up climate action. SPM.7
• There is a rapidly narrowing window of opportunity to enable climate resilient development. SPM.6
• Limiting warming to 1.5°C and 2°C involves rapid, deep, and in most cases immediate greenhouse gas emission reductions. SPM.5
• Risks are increasing with every increment of warming. SPM.4
• Future climate change is projected to increase the severity of impacts across natural and human systems and will increase regional differences. SPM.3
• With every increment of global warming, regional changes in mean climate and extremes become more widespread and pronounced. SPM.2
• Adverse impacts from human-caused climate change will continue to intensify. SPM.1

Here is a direct link to the high resolution Figure SPM.7 chart in the AR6 Synthesis Report.

"Solar, according to the IPCC report, can deliver more emission cuts than any other technology by 2030, when the world needs to have cut its emissions by at least half if it is to have any chance of capping average global warming at 1.5°C. Solar and wind together offer nearly ten times the emission cut potential than nuclear, and 20 times that of carbon capture."

"Worldwide, climate change sceptics fall into three main categories.

First of all, you have the climate denialists, who are economically motivated. For example, fake activists are paid by the fossil fuel industries with the aim of delaying action against global warming. This has already been extensively documented.

Then there are the political climate sceptics, who reject the reality of global warming and denigrate the measures proposed to address it, mainly to undermine the political opponents who support these initiatives. They are not necessarily interested in global warming as such. The US elections saw a resurgence of climate scepticism because it provided an opportunity to attack the Democrats' environmental agenda.

And finally, a third category consists in geopolitical climate scepticism, originating in countries with totalitarian regimes. For these governments, such as the Kremlin, the climate crisis is a chance to divide populations and weaken democracies, as I explained in my book Toxic Data. For instance, we know that one of the strategies Putin uses to gain geopolitical influence is to carry out subversive operations on the social networks in a bid to weaken the democracies. His goal is to exacerbate internal divisions in order to alter social cohesion and ensure that the attention of governments is taken up by internal conflicts, or even, where possible, that these governments are themselves delegitimised.

The upsurge in climate change denialism that we've seen since the summer of 2022 appears to originate, in large part, in this third current.

We've come across accounts that initially spread dissension about Covid-19 vaccines, before relaying the Kremlin's propaganda about the war in Ukraine, and eventually defending climate-sceptic theories. 60% of the climate-denialist community active in 2022 took part in pro-Putin digital campaigns."

Investigating climate sceptics’ disinformation strategy on Twitter 03.13.2023, by Sebastián Escalón

> country that has completely switched over to renewables

In predicting the future of renewable electricity generation it is very important to base our predictions on future renewable prices, not on historic prices, because renewables are getting much cheaper relative to other sources, such as coal, gas and nuclear. Existing installed generating capacity was all purchased at historic prices when renewables were more expensive.

Looking into the future we can see the US is switching to a fully renewable electricity grid. This transition is happening even with current renewable prices. The 2023 planned additions and retirements according to the EIA are

A massive increase in solar pv, wind and batteries, and a massive decrease in coal. Not much change in natural gas, but we know from Swanson's law the volume manufacture of solar pv will continue to bring down the price and lead to a spiral of increasing manufacturing capacity and reducing price. Similarly, wind turbines are getting cheaper but at a slower rate. These bite into the profitablility of natural gas electricity generation by making the gas plants into peaker plants, which are approximately twice as expensive per kWh as continuously operating plants. The gas peaker plants are then more expensive than, and have difficulty competing with, grid-connected batteries. Batteries are also increasing in volume and falling in price partly because the auto industry is going all-in with BEVs which already have 14% of the global market in 2022. BEV sales are increasing at a conservative estimate of 30% per year which means they also represent a second significant threat to the oil and gas industry by reducing gasoline consumption - gasoline being the major component of crude oil.

The wikipedia list of countries by renewable electricity generation needs updating from mostly historic 2016 numbers, but may give sources for its country-related renewable electricity data. The changes since 2016 might be similar to Australia which has significantly increased solar and wind generation in the last 10 years. (The chart of total energy consumption by Australian state shows a less optimistic picture of the transition from fossil fuels - the need to electrify more of the world's energy usage.)

This economic case for solar PV and wind turbines now makes the transition from fossil fuels inevitable, and as manufacturing capacity for solar pv and renewables is built the transition will become increasingly rapid.

"If you look at the total world economy, it’s just under $US100 trillion. So if this was spread out, say over 10 years, it would be 1 per cent of the global economy."

Inspired by your answer, I imagine we might also consider the fact that m1 and m2 are not perfect rigid point masses. Each is a spring-connected set of points of mass - molecules and atoms with intermolecular forces - spread in a volume. The individual points of mass will exert different gravitation forces on each other and have time-delayed spring forces transmitted through the network of points. I guess we can ignore the speed of gravity when doing a rough approximation.

I was given a bit of information in this thread about radiation from burning coal.

> approved and certified in systems that dont exist yet - like .. or even the electric motors

The UK already has fully certified electric aeroplanes with batteries and electric motors in commercial operation. They are quieter and cheaper to run than the fossil fuel equivalents.

Other possible ways of decarbonizing planes are sustainable aviation fuel and electric aircraft. Hopefully somebody will solve it for us soon.

ZeroAvia's flight is part of the UK Government-backed HyFlyer II project, which aims to develop a 600kW powertrain to allow zero-emissions flight for 9-19 seater aircraft, and is targeting a 300 nautical mile range. The flight was conducted under a full Part 21 flight permit with the UK Civil Aviation Authority.

The company aims to be serving commercial flights with the technology by 2025. It also aims to scale up the technology to larger 90-seater aircraft, with "further expansion" into narrowbodies in the next decade, they said in a statement. By 2027, they aim to be able to power a 700-mile flight in a 40-80 seater aircraft.

The speed of sound is the speed at which changes in pressure move through the air.

If a bullet is shooting through the air faster than the speed of sound it is said to be "supersonic". (The word "supersonic" literally means above, indicated by the Latin word "super", like "superior", stuck on the front of the word "sonic" meaning sound. "Sonic velocity" means at the speed of sound, and "subsonic" means below the speed of sound, Latin word "sub" meaning below stuck on the front of sonic, like submarine is below the surface of the sea.) A supersonic bullet is moving faster than the sound pressure changes move through the air. So you will not hear the bullet before it hits you.

All the energy in the sound pressure changes that are not going forward from the supersonic bullet has to go somewhere else instead. The pressure changes go into sharp changes in pressure between the air in front of the bullet and the air behind the bullet. These sharp changes in pressure are called "shocks", and they happen when the bullet reaches the speed of sound. Subsonic bullets do not create shocks because the energy can move forward in front of the bullet - you will hear a subsonic bullet before it hits you.

The physics answer has been given, but since this is asked under an Engineering tag it should also be noted that there is something called wind engineering setting standards for the wind speed a building should be able to withstand, and there will be turbulence effects from the neighboring buildings and the height of the window above ground which modify the local wind velocity compared to the wind speed you see on a weather map.

Yes, I agree, it would be good to move traffic onto electric trains. There are also short-range electric aircraft which could reduce the aviation emissions.

> 17% of Australian emissions come from transportation

The latest transport emissions are about 18% of Australia's total emissions and that breaks down to about 10% of total emissions are attributed to light vehicles and trucks and aviation are a large part of the 7% remaining. Train PDF emissions seem very small compared to trucks or aviation.

This report PDF demonstrates that an 81% emissions reduction is achievable by 2030. This requires immediate and large-scale actions, prioritising short-term ambitious targets for already-available technologies. The report outlines a five-year deployment plan, identifying the most impactful technologies. It will create 195,000 jobs and put Australia on the IPCC scenario SSP1-1.9 (for 1.5 degrees of average global warming). Six existing technologies will do the heavy lifting: solar panels, wind turbines, batteries, electric vehicles, heat pumps, and hydrogen electrolysers.