Cars and trucks account for nearly one-quater of all greenhouse gas emissions in the USA, and 1/5th of emissions in the UK, which makes it easy to see why such a focus is placed on them in lowering carbon emissions. Electric vehicles are the majority of this discussion, and the conversation is often confused by some over-emphasizing their importance, and others over-emphasize the downsides without really explaining the science behind it all.
In countries such as Sweden, which gets most of its electricity from renewable sources, and France, which is largely powered by nuclear, the CO2 savings from using electric cars reach as high as 70% over their conventional counterparts. In the UK, the savings are about 30%. However, that is likely to improve further as electric vehicles grow even more efficient and more CO2 is taken out of the electricity generating system.
Basically, throughout 95% of the world, the lifecycle emissions of an electric car (manufacturing, shipping, running, and scrapping) are lower than fossil fuel cars, unless the country is incredibly coal heavy like Poland. The U.K. still has a few coal plants, but the more that close, the lower the carbon emissions become whenever the car is charged.
In the UK in 2019, the lifetime emissions per kilometer of driving a Nissan Leaf EV were about three times lower than for the average conventional car, even before accounting for the falling carbon intensity of electricity generation during the car’s lifetime. -- Source:"How electric vehicles help to tackle climate change", Carbon Brief
An electric vehicle, once purchased, can be powered by an increasingly efficient source of energy. The same obviously cannot be said about petrol cars, which are always going to be stuck on petrol. There is a common belief that modern cars are more efficient than older cars, but cars are becoming heavier and less efficient over time.
Most electric car batteries last longer than people think, for example all batteries in electric cars sold in the U.S. are covered under warranty for at least 8 years or 100,000 miles. Kia covers the battery packs in its electric cars for 10 years/100,000 miles, while Hyundai goes even further bumping it up to lifetime coverage. Free replacement batteries are provided if they fail to meet those requirements.
When the battery packs in a lithium-ion-powered vehicle are deemed too worn out for driving, they still have up to 80 percent of their charge left. So before they ever get to a recycling center, these batteries are used to prop up the grid, especially alongside energy sources that may not be quite as steady, like wind or solar power. -– Source: How Stuff Works
So should everyone replace all their petrol cars with electric cars? Well, kinda? Replacing every single petrol car with an electric car right now would be a rough time, because what are we doing with all the old cars? Scrapping them is one option, and is certainly the right thing to do when a car has reached the end of its safely functioning lifespan, but doing this prematurely would be asking people to throwing money away, and the carbon emissions from melting down all these cars prematurely would be wild.
Converting petrol cars to electric would be a better option for most modern cars, as this reduces the amount of waste as only the engine is replaced, most of which can be melted down anyway. This is happening in Amsterdam with petrol/diesel boats being banned from the city in 2021. They don't need to throw away the whole boat, they just replace the engine with an electric one. The process is more complex for cars, but there are specialist garages around the world who can help.
Even better is a change of thinking to remove our reliance on personal cars. Some countries have 7 or 8 cars for every 10 people, with American households having on average 2-3 cars. Why not sell 2-3 of them, buy one electric car if you absolutely require a car due to poor bike infrastructure around your home, and use the rest to buy a bicycle, electric bicycle, or even an electric motorbike!
Reducing the number of personal cars cluttering our streets means more room for bike lanes, pedestrian areas with outdoor seating for restaurants, and car parks can be replaced with public parks, more housing, or a myriad of other more useful things.
Beyond that, some countries are running out of low hanging fruit to make giant cuts to their CO2 emissions. The UK closed most of its coal plants to make its 29% cut between 2010-2019, now the largest sector is transportation, and what better way to help cut those emissions as quickly as possible by switching to active transportation instead of switching to something which 30-70% better. Active transportation is 100% better, and the we cycle the safer bike infrastructure gets for everyone.