Altruism is important, but so is basic personal responsibility - and basic personal responsibility arguably takes precedence. For instance, the cost of feeding myself and my family is important, but it's not altruistic. This is necessary for personal reasons. For obvious reasons, I budget for altruism separately from my personal expenses. That means that my kid's clothes and school costs, my vacations, my health care costs, and my retirement savings are all included in personal costs.
Perhaps the world would be better if some of these weren't personal expenses - if my health care and retirement were covered by government programs, and these were provided universally. A functioning economy can either include these as personal expenses, or include them as part of a government budget. That structural choice changes my personal responsibilities to my family by moving the cost of educating my children from my personal budget to the government's budget. It might even change my moral responsibility to help those in my country. To the extent that I have a responsibility to the world's poor, however, the governmental choice to cover or not cover my healthcare is irrelevant.
According to most economic theorists, a market economy can similarly choose the location for the costs of externalities. It is often more efficient to put externality costs on the people who generate the externality, rather than imposing those costs on those affected. This is why governments tax cigarettes and ban dumping waste into rivers. In some cases there are reasonable arguments that such externality costs shouldn't be internalized, since measurement is infeasible or easily corrupted, or because costs of enforcement are high, or because personal liberty or privacy is unduly affected. In each case, the governmental decision affects my personal budget in ways that have moral consequences.
Sometimes, this can be enforced legally, but not always. In cases where externalities are not fixed by governmental action, individuals still have a personal responsibility not to harm others. In cases where property rights do not compensate for the externalities, individuals still have moral responsibility for the consequences of their actions.
The argument to avert harm to others is not an altruistic claim. Instead, it is simply noticing a part of my basic personal responsibility. Given that, there is a glaring gap in governmental response to a specific externality that I have a personal responsibility to address - my carbon footprint.
If it were unduly burdensome to understand or to offset my personal impact, there might be a case that I am not required to go to great lengths to fix the problems. In fact, quantifying my impact (and that of my family) is remarkably easy. Similarly, if offsetting my impact was incredibly burdensome, I might be excused from doing so. But even using traditional, ineffective offsets, the cost for the average US household is only a few hundred dollars.
Instead, however, Vox's reporting found that I can offset my family's carbon yearly footprint far more effectively, and cheaply. There is a clear case for a moral responsibility to give at least the estimated $6 it would cost to offset the average family's carbon emissions by giving to the most effective program found. Given the uncertainty around the impacts of carbon offsets, I can afford to be risk averse and give 5 or 10 times that amount, and spread my donation across a couple of the most effective programs to account for the uncertainty. But even for those who cannot be quite so generous, or don't have much time, they can still take 5 minutes to just give the money.
Thanks a lot for writing this up - motivated me to donate to do offsets this year!