A "watermelon" is a nominal environmentalist who uses a "green" agenda to push a left wing and/or special interest agenda.
Today, Global Warming is used as a Trojan Horse to advocate radical left wing policies. The changes in the former USSR, Eastern Europe, China, and other nations prove conclusively that Marxism/communism is a terrible idea, induces poverty and human rights violations, encourages dictatorship, and will spread whenever it's given a chance.
However, in spite of their failures and terrible record, the radical left hasn't abandoned their ideas. They realize communism doesn't sell very well, so today they use populism, and pseudo-environmentalism to try to achieve their goals.
An example of the way a seemingly innocent agenda is subverted is the UN Development Program's global warming agenda and tactics, including its "Climate Change StoryTelling Contest". In their own words:
"UNDP story telling contest on climate change aims to contribute to raising public awareness on the negative impacts of climate change on people and communities as well as on the opportunities and solutions seen in actions by individuals and governments in vulnerable developing countries.
As we seem the UNDP contest rewards young journalists with a trip to Paris, and widespread exposure if they write the appropriate climate change propaganda. UNDP is also receiving financial aid from organizations such as Oxfam, whose agenda is set out in their own statement:
"Oxfam will support the Voice2Paris global storytelling contest launched in August by the United National Development Programme (UNDP) by providing three additional fellowships for participating journalists to cover the UN Conference on Climate Change, COP21, in Paris in December. Oxfam’s contribution to the contest aims at encouraging journalists’ participation in climate change reporting and raising public awareness of climate actions.“The contest is a fantastic opportunity to create awareness of the harmful impacts of climate change on communities, and of potential opportunities in climate-vulnerable developing countries. This is also a great opportunity for young journalists to strengthen their perception of climate change and to frame it not merely as an environmental issue but also as an issue of social justice and poverty alleviation” said Wang Binbin, Manager of the Climate Change and Poverty Team, Oxfam Hong Kong."
The radical watermelon agenda is also pushed from within the USA and EU governments. For example, here's a quote from John Holdren, President Obama's "Science Advisor" (from his book “Ecoscience: Population, Resources, and Environment”):
“...organized evasive action: population control, limitation of material consumption, redistribution of wealth, transitions to technologies that are environmentally and socially less disruptive than today’s, and movement toward some kind of world government”