"School mathematics has traditionally organized some of its applications around the needs of the moment."
When I first started reading this I wasn't sure of the connection, ecological sustainability and math education seemed like a bit of a stretch, but this makes sense. Throughout human history, math problems have always been posed in terms of current needs. For example the Egyptians dividing up loafs of bread for multiple workers. I'm not sure in what form problems of the future might look, but posing climate problems as math problems might be one of the first necessary steps in guiding the future of humanity to address those issues. People generally ignore these issues or brush them off, but it's the future generations that'll suffer, so why not make them more aware now, and be comfortable with necessary talks.
"Even though sustainable mathematics education is motivated by urgent issues of survival, it need not adopt the pessimistic tone of many writings on ecology."
This is what I meant. People avoid these needed conversations because it makes them feel bad. If you grow up with these talks, and their in the open, it gets easier to deal with the actual problem at hand. I don't think we need to avoid the pessimistic tone entirely; I think it's important to know how serious this problem is, but it should also be delivered evenly with the knowledge that something can be done.
