The best option (other than bicycle, for those of you already in Amsterdam!), is to travel by train..
Eurostar now have a new route direct from London to Amsterdam:
The Thalys rail network connects Amsterdam with Paris, Brussels, Antwerp and Rotterdam:
Trainline have times and tickets for trains to Amsterdam from right across Europe:
Again, driving isn't great for the planet, but its significantly better than flying! If you can drive, do consider halving your carbon footprint by sharing the journey with a fellow Summit attendee.
Or use one of the more popular car pooling apps to find someone heading the same way that you could share your car with:
No alternative but to fly? Consider off-setting the carbon your journey has used by using one of these offsetting websites to give something back to the planet:
KLM have a program called CO2Zero. It works by adding a fee to your flight at point of booking. Other airlines will offer something similar so do check before you book:
The Gold Standard is a website that allows you to choose the kind of project you donate to, to offset your carbon emission: