A perennial debate has once again emerged at this year's U.N. climate talks: in order to make a meaningful dent in carbon emissions, do we need to invest more in R&D or more in the deployment of existing technologies?

The world's largest investment bank, Goldman Sachs, is now weighing in.

In a report issued this week, Goldman concluded that the most effective mid-term solution for lowering carbon emissions -- and making money -- is backing existing technologies that are ready to scale. In fact, analysts at the bank believe that the majority of investor dollars should go to only four technologies: solar photovoltaics, onshore wind, LED lighting and electric cars.

"We believe investors should focus on this set of front-runners with the potential to shift emission trajectories and reshape competitive dynamics on a five- to 10-year view," wrote the group of analysts.

These four areas collectively make up the vast majority of the low-carbon investment opportunity, which Goldman values at $600 billion a year.  

"There are many other low-carbon technologies that either lack the scale or the momentum to drive significant large-scale global change in 2015-2025. They include: (1) mature technologies with relatively slow, stable growth trajectories; (2) early-stage technologies that have growth but still lack scale; and (3) technologies that are gradually losing regulatory support," wrote the Goldman analysts.

Those technologies include biofuels, nuclear, large hydro, carbon capture, marine power and offshore wind. 

The report's findings contradict the conclusions of some of the world's top tech billionaires, who believe far more money needs to flow into early-stage technologies in order to radically improve upon the cost and performance of intermittent renewables. This week, those billionaires formed the Breakthrough Energy Coalition and pledged to spend far more on cleantech R&D.

“The existing system of basic research, clean energy investment, regulatory frameworks, and subsidies fails to sufficiently mobilize investment in truly transformative energy solutions for the future,” states the coalition's website.

It's not surprising that the bankers at Goldman Sachs, who are focused on profits, see the world differently than tech CEOs who are willing to fund risky ideas that may not pay off for decades. 

Goldman isn't betting against technologies outside of its four preferred areas. Instead, the bank is focused on the "greatest market dislocations" between now and 2025. Solar PV, wind, electric cars and LEDs are the technologies best prepared to erode fossil-fuel consumption within that timeframe.

By 2020, wind and solar alone will be producing the oil equivalent of 6.2 million barrels per day -- adding more energy to the global supply, as American shale producers did from 2010-2015.

"We expect wind and solar to account for over half of (capacity-adjusted) new installations in electricity generation capacity by 2025. In energy terms, this now puts the phenomenon of solar PV and onshore wind on par with U.S. shale oil production," wrote the Goldman researchers.

In order to limit the global temperature increase to 2 degrees Celsius, climate scientists say greenhouse-gas emissions need to peak by around 2020. On their current trajectories, solar PV and wind could eliminate 5 gigatons of carbon dioxide emissions each year by 2025 -- or about one-sixth of the emissions pumped into the atmosphere globally in 2014. 

The Goldman report only looks at macroeconomic trends. It does not consider the limitations of conventional renewables due to grid constraints, land-use conflicts or performance. Some argue those factors will create an upper limit to the amount of wind and solar that can be deployed on grids around the world. 

A study released by MIT this week backs up Goldman's conclusions about installation and cost trends. After examining the climate pledges of countries around the world, MIT researchers found that conventional wind and solar would be the biggest beneficiaries of carbon reduction efforts. 

According to the report, global solar installations will grow fivefold and wind will grow nearly threefold by 2030 under voluntary carbon-reduction plans established by countries. This could result in a 50 percent cost reduction for solar PV and a 25 percent cost reduction for wind by that date.