Well, as they say, 'the science is in' and the answer is increased atmospheric concentrations of 'greenhouse' gases (CO2, methane, CFCs etc.) causes a larger 'greenhouse effect' and increases global temperatures. In fact this was already an established fact when I studied environmental chemistry back in the early 1980s. There was a bit of debate about the cause(s) of the increased levels of CO2 in the atmosphere, and how big an impact it has on climate, but it turns out that the obvious cause (us) is also now pretty much beyond doubt. Unless of course you're a climate change 'skeptic' that chooses not to allow evidence to affect their chosen/indoctrinated beliefs (similar to flat-earthers, moon landing skeptics, and, funnily enough, religious 'believers') - so you pretty much have to accept that global warming is artificial, and not a natural variation in climate. So it isn't magically 'go away' unless we do something about it.
But the key driver is actually the sheer number of 'us' on the planet. It was fine for a small percentage (a few hundred million people) to want/have cars, energy-intensive (industrial) jobs, and all manner of labor-saving and entertainment electrical devices, but when you combine a natural desire for nearly everyone to achieve similar living standards with an massive increase in the total population, the problem becomes much more intractable.
It is interesting to simply compare global greenhouse gas emission levels with global population - it can be seen that from 1850-1980 the correlation was obvious and unbroken. It is only from 1980 onwards that there has been some slight reduction in the trajectory of carbon emissions compared to population. Unfortunately any reduction in per capita emissions has not been enough to solve the problem of increasing population. I've added the black line showing (roughly) where carbon emission levels would have gone (simply driven by increasing population and development) if not for the mitigation achieved so far (more energy efficient technologies, non-fossil-fueled energy sources etc.).
Well, as can be seen from the population chart, population growth rates peaked in the 1950s, and are slowly trending down towards a projected 0.1% by 2100 (still almost twice the rate of about 0.04% that applied prior to 1700 - before infant mortality rates plummeted). So, in the very long term, stabilizing global population would stabilize carbon emission levels even in the absence of any per capita reduction in carbon emissions. And, if/when the entire world is developed, with high living standards, literacy levels, and gender equality, fertility rates might even drop below replacement level globally, allowing the global population to slowly decline to a more sustainable level (2 billion? 5 billion? 10 billion?). Unfortunately, the global population at the time of the Paris Climate agreements was around half what it will be by 2100 -- so the global average per capita emission level will have to be halved simply in order to 'tread water'.
But as global population will continue to rise for many, many decades, the only way to address global warming is to reduce per capita carbon emissions by a massive amount. Enough to more than offset the increasing population. Comparing per capita levels for different countries reveals a number of things:
1. Developing countries (China and India especially) are rapidly increasing per capita emissions towards that of developed countries. Combined with substantial (and growing) populations, these countries need to be able to achieve development without hitting the same levels of per capita carbon emissions of the USA. So, building a whole lot of fossil-fueled power stations and providing the population with petrol-powered cars isn't really sustainable. China already has higher per capita emission levels than Europe. So, while developed countries have been cutting their per capita emissions, the global average per capita emission level hasn't changed very much:
2. Developed countries that are a) large and b) high per capita emitters, need to initially reduce their per capita carbon emissions towards 'best practice' amongst developed countries.
3. After that, the 'developed' countries (including, by then, China) will need to get their per capita emissions to a much lower level than what is currently 'best practice'. While there are some constraints (not all countries can rely on geothermal power like NZ or Iceland, and not all developed countries want to incorporate nuclear power in their energy mix like France) improved renewables technology should make it possible for developed countries to decrease their per capita emissions significantly in the next few decades.
Unfortunately improved per capita emissions in the already developed countries can't solve the problem. Getting to 'zero emissions' in the UK or EU will be helpful, but this will be overwhelmed by the population growth and development effects of China and India. It is even more important for the developing countries to be aiming for per capita emission levels significantly lower than those achieved by current 'developed' countries. The following chart doesn't make this seem too likely - China, while still only 56% urbanized already has higher per capita emission levels than the EU (which is 74% urbanized). As India increases urbanization (from its current level around 34%), it is also likely to overtake Europe and the USA as a source of carbon emissions.
Although it is now, apparently, a climate 'emergency' the most obvious (and effective) means to slow the rate of global increase greenhouse gas emissions (actively working towards ZPG and slowing the rate of development in underdeveloped countries) are 'off the table'.
The current debate regarding 'what to do' about climate change seems to be a nonsensical reversal of the 'pareto principle'. Rather than focus on actions that could/should be taken to impact on 80% of future increases in carbon emissions, the focus seems to on actions that can only have a trivial impact on the total level of global carbon emissions...
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