What practicable alternative do I have to not fly around the globe to present a poster/pollute environment?
If you want to build a strong career and be noticed internationally, then you probably have little choice but to attend international conferences in the present situation.

Longer term, however, you can try to work toward a world of virtual conferences rather than ones requiring physical presence, though these have tradeoffs. But the current and future communication technologies will make them more practical as time goes on. 

But, there are other things to consider than just the carbon-cost of air travel. First, the plane will fly anyway even if you aren't on it. Collective political action can work toward a world with less air travel, of course, but an individual has little real effect. The carbon cost of air travel is amortized over a large number of people, of course, and if they don't fly they still don't have zero carbon usage. Depending on your normal activities and travel (to work, say) needs, a week long conference across the pond may not cost you much in carbon, if there is little travel at the conference site and things are centralized. 

But if you do travel, there are things you can do to better utilize the benefits of the costs you incur. Making arrangements before you go to work with international colleagues for a few days before or after the conference helps amortize the cost over a larger set of activities, chaining the cost/benefit ratio. 

You can, of course, think globally about your carbon usage and find other ways to reduce your footprint so that, even as an individual, you have a minimal negative impact. 

Also, communication technologies aren't carbon-free. The trade-off computations are very complex, in fact. I just read a report that says that streaming movies, for example, has a higher carbon footprint that pressing and distributing DVDs due to the energy required by servers (a continuous cost) vs the pressing/distribution cost (discrete). 

Our climate issues are a result of bad choices by society as a whole, making better solutions difficult or impossible to employ by individuals. An individual has little effect, especially if they are conscious of the problems and take steps to limit personal use where possible, but also to demand better overall solutions of governments and other large institutions.