Rather than focusing on the biomass of a human being, I think it would be appropriate to consider the mass of all the things connected to an industrialized human's life: house, car, roads, stores, school, mono-cropped fields, power plant, sewage treatment facility, dead zones in rivers and oceans, electronics laced with heavy metals, etc...
I don't deny that human beings have had an effect on ecosystems. There are definite negative ecological issues caused by anthropogenic activity of certain kinds, and I think that's what the environmental movement should be focussing on rather than CO2 production, which if anything is ecologically beneficial because it is increasing plant biomass at the moment.
Additionally, the measures being taken to combat the highly exaggerated issue of anthropogenic CO2, supported by the Green Movement, are contributing to environmental problems. WRT the plant monocultures you bemoan, these days that is in large part due to the cultivation of biofuel sources, which has led to significant forest clearing and doesn't actually do anything to address the putative issue of anthropogenic CO2 production. This is something that some of the more enlightened Greens are increasingly recognising. Even Al Gore has admitted that this policy has been a mistake. The planting of biofuels has also led to reductions in land available for planting food crops, which has in turn raised food prices, most affecting the poor.
What about the mining of Neodynium for the magnets in wind turbines? This has had disastrous environmental effects in China. Wind turbines themselves have led to a lot of habitat destruction: they require extensive concrete foundations and kill millions of bats and birds, including rare raptors.
I could go on and on. What quite a lot of so-called environmentalists don't realise because they don't do research in areas that the Green Movement prefers to sweep under the carpet, is that the whole CO2 farrago might have been designed expressly to screw up the environment. The movement is in denial about the fact that it has come to promote the opposite of what it claims to stand for.
People want for us to have a healthy environment? Good. So do I. Then they should take a critical look at the heavily politicised Green Movement and recognise that it's ignoring the important issues and causing more harm than good. The real issue it's pushing isn't the environment: in the end, it's the redistribution of wealth.
Personally, I think that morally speaking that's a laudable aim if pursued in a sensible way. All indications are that the wealthier countries are, the less they tend to damage and pollute the environment. To become wealthier, they need copious affordable energy. The most effective way to provide that at the present moment is probably through thorium-fired nuclear power (though that may not always be the case: it's possible that there will be a breakthrough in some other area, such as LENR). But guess what: the great green blob is ideologically opposed to nuclear power as well as conventionally mined oil, coal, and gas.
It wouldn't be so bad if wind and solar were practical alternatives. But actually, they're pathetic and unreliable sources of energy that don't even succeed in reducing CO2 emissions. They have to be backed up by conventional power generation because the wind doesn't always blow and the sun doesn't always shine. These inefficiencies offset any benefits they are supposed to bring.
Finally, a more philosophical point: human beings are natural organisms no less than any others on earth. They have effects, some positive and some negative, and the same can be said for all organisms that exist or have ever existed on earth. Around 2.5 billion years ago, so the current story goes, there was the great oxidation event, when the dominant chemotrophic organisms were severely affected by ones that could generate oxygen through photosynthesis.
If I'd been an intelligent chemotroph back then, I'd have bewailed the disastrous effect of these newcomers on my environment. They completely overturned the status quo, and many events since then have continued to radically change ecosystems. Human beings are natural, and unlike any other organism we know of, are capable of having conscious regard for other organisms. They aren't perfect in this respect, and some of them love to beat themselves up (or maybe the "others" whom they tend to blame without looking in the mirror?), but what other organism would even be capable of questioning its own motivations and effects?
I believe that, lying behind a lot of the angst is an unhealthy degree of hatred of humanity: and that may well be just a stage in the evolution of human society that we need to overcome. In one way or another, balance will be achieved; the earth will carry on regardless and is immensely bigger and more influential than we can ever be. Let's stop being so pessimistic and have a little more compassion for our own species.