Please don't answer without citing scientific research which has a general consensus or article from a reputable source based on verifiable research citations.
I recently read this article about a 2017 PNAS study from Griscom et al (link below) that "estimated natural carbon solutions (NCS) (essentially ecosystem regeneration) have the potential to provide up to 37 percent of the CO2 mitigation that we'd need through 2030, for a 66 percent chance of holding warming to below 2°C."
More specifically, what is the most cost-effective solution? Most geoengineering solutions have significant risks or unknowns which greatly reduce their benefit. Reducing fossil fuel use immediately will reduce short-term economic output with replacing with renewables and alternatives. Maybe simply paying drivers incentives to use alternative vehicles is most-cost effective, as transportation is the largest percentage of greenhouse gas emissions in the US (EPA).
So what is the most cost-effective and how much money would it take to get it done? Global energy-related CO2 emissions were 33.1 Gt CO2 in 2018 according the [IEA report]. Apparently from the Griscom study, 10 USD MgCO2e−1 is the "current cost of emission reduction efforts underway and current prices on existing carbon markets" from 2017. 1Gt = 1Pg = 1000Mt = 1000Tg = 10^9 tonnes metric, and 1Mg = 1 tonne, so our CO2 emissions cost us 10*10^9 or 10 billion USD per year. For reference, global GDP is at US$84.84 trillion for 2018, meaning reducing our entire emissions to zero would cost all of 0.0001% of global GDP.
As as corollary, if all benefits of NCS to uncalculated ecosystem services were included, would they be the most cost-effective, and what is the average ROI on research investment to calculate them?
To the Mods: I've changed the title to match the narrow question specified in the body of the question. I've provided an answer that is very specific using calculated figures from cited references. What else would you like to make the question more narrow?
Griscom et al, https://www.pnas.org/content/114/44/11645