The success of those carbon offset efforts is tough to measure and there appears to be growing scepticism about their overall effectiveness, with mounting arguments cautioning that emissions from certain offset projects need to be kept in check.
Delta and JetBlue have had carbon offset programmes in place since 2007 and 2008 respectively, and they were later joined by Alaska and United. Simply put, carbon offsets programmes allow passengers to make purchases that support sustainability projects in order to neutralise the carbon emissions of their flights.
Both Alaska and JetBlue partner with carbonfund.org for their offsetting programmes, while Delta's scheme supports environmental initiatives in Belize, Chile, and the US state of Virginia. United partners with Conservation International for its carbon offset purchases.
Those airlines offer passenger calculators to determine how much to invest in offsets, largely based on the length of flight.
Over the years, airlines have not really publicised the results of their carbon offset schemes. But in Apr-2019, to mark 'Earth Day' in the US, carbonfund.org stated that since 2008 JetBlue employees and customers had reduced more than two billion pounds (907 million kg) of carbon dioxide emissions.
Carbonfund calculated that that that was the equivalent of reducing greenhouse gas emissions from more than two million miles driven by the average passenger vehicle, or carbon sequestered by more than 15 million seedlings grown over 10 years.
In addition to offering passengers the opportunity to purchase their own carbon offsets, Delta has declared that it is the only major airline to cap carbon emissions voluntarily at 2012 levels by purchasing carbon offsets and since 2013 it has voluntarily purchased more than 12 million carbon offsets, which is equivalent to emissions from 1.7 million cars or the electricity use by nearly two million homes in one year.
The effectiveness of carbon offsets has always drawn a certain level of scrutiny, since some sceptics argue that the offset projects actually create carbon emissions.
A recent article in Forbes stated that a passenger balancing the carbon emitted by a flight through supporting the planting of trees in South America, "does not involve solely the solitary act of placing the tree in the soil". Those trees must be purchased from a supplier, transported to a warehouse, and driven to a site that needs to be cleared out, the article explained. "All these actions produce their own share of carbon emissions, which are not always taken into account."
A 2009 New York Times article noted that Responsible Travel had cancelled the carbon offset programme it launched during 2002. In the report, Responsible Travel remarked that carbon offsets had become a "magic pill…it's seductive to the consumer who says, 'It's [USD]$4 and I'm carbon neutral, so I can fly all I want'".
The Forbes article highlighted a quote from Nature that sums up the complexity of carbon offset programmes and proclaimed: "…for an offset project to be genuinely low-carbon, it must guarantee that it does not stimulate further emissions over the subsequent century".