Reducing carbon footprint

What technology could be developed earlier that would result in a lower carbon footprint society today? Also other helpful green technology would be welcome.

For example I think algae fuel and fertilizer could've been ready years ago if we invested in it earlier. Also biochar could've been done before the 1900s since it's just factory made terra preta.
 
Widespread Nuclear power would go a long way in helping since it would negate the need for power derived from coal and oil. The public has generally been ignorant about nuclear power, thinking they're poised to break down and that any problems will result in a mushroom cloud, which stagnated the technology and only in recent years has the idea popularly come back up about building more nuclear power plants.
Ethanol and the electric car would help as well. The first vehicle ran on ethanol, if I'm correct, but because gasoline was so plentiful and inexpensive, it dominated.

Sorry if this unleashes Pandora's box, but what about preventing harmful technologies? For example, leaded gasoline, DDT (or rather, the unchecked and dangerous over-usage of), etc?
 
A lot of the technologies used for the purpose are old hat and could have been implemented decades ago, had there been a call for that. The problem is that there was no need perceived, and even today I suspect the threat of rising energy prices and uncertain supply is far more important as a driver of technology than the desire to reduce carbon emissions.

That, I think, would also be the way to get there (even before the problem of carbon emissions becomes apparent, back in the days when the headaches were lead, sulphur dioxide, or soot). Make energy less dirt cheap, or less reliable. The twentieth century was an unbelievable time in retrospect - governments spending huge amounts of money and passing coercive legislation to create a centralised energy infrastructure dependent onscalable sources, exploiting oil reserves worldwide and subsidising energy use. A little less oil availability, or less centralised power grids, could have done a loit. 'Alternative' energies were always competitive on the small scale, and wind and hydropower could have played a role much earlier. More importantly, economical use of energy could mean huge savings. If you look at the difference between postwar American and European gadget culture, you can see what was possible in terms of economies even in the 1950s. Of course, Europe gave up on that when the Trentes Glorieuses really hit, and America didn't really get into it until the oil crisis. A Cold War era with higher energy prices will harness engineering minds to producing energy-efficient solutions much earlier. Insulated homes will become standard, and perople will invent systems of ventilation ducts that cool the home or keep in the warmth, depending on how they are configured, rather than blowing electrically cooled or heated air into each room. Solar collectors for water heating were known well before pholtovoltaics, but oil was cheaper. You'd see a market for things like cooking boxes, insulation packs for ovens, smaller or multiple-unit refrigerators, smaller cars, perhaps even multifuel cars with the option to burn ethanol or wood gas or vegetable oil. But aove all, you'd see people burning a lot less coal and oil. Locally generated electricity (wood, biomass, solar, wind, hydro, geothermal, nuclear) may well seem like an economy measure in comparison rather than an expensive, but politically desireable solution.
 
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