As clearly shown by the 2019 elections for the European Parliament, in the last decade many European Green parties have strongly increased their votes, but above all their relevance in the national party systems. Several of them passed for the first time the Pedersen’s  fourth threshold, entering in government coalitions in Austria, Denmark, Iceland, Lithuania, Luxemburg and Sweden, joining veterans like the Finnish Greens and getting two Presidents and two Prime ministers. Other minor parties passed the third threshold, debuting in national or European Parliament.
A closer look shows that Greens perform very differently in the European macro-regions, with stronger and more relevant parties in the North (and the West) and weaker and marginal ones in the East and the South. What are the origins and the causes of this gap? The paper will try to provide a first answer considering four external and three internal factors. The former are:
a) Spread of post-materialistic values in the population [Inglehart 1977, Abramson 2014];
b) Rising of the environmental themes (especially the climate change awareness) in the media and public’s agenda;
c) The worsening crisis of mainstream parties which has nearly completely ‘defrosted’ Rokkan’s cleavages, offering to Green parties the possibility to exceed the limit of their traditional left-libertarian voters [Kitschelt 1990];
d) Electoral laws (in several cases differentiated for national and over/sub-national levels).
On the other side, three internal factors could be also relevant:
e) The replacement of the original ‘amateur-activist’ party model [Kitschelt 1989, Poguntke 1993] with the ‘professional-electoral’ one [Panebianco 1988];
f) Gender equality policies. Green parties introduced balancing rules both for internal and parliamentary positions in the 80’s. After the European Green Party’s foundation in 2004 the co-leadership held by a man and a woman, originally adopted by German ‘Grunen’ in 1993, has spread across the continent;
g) Post-ideological alliances: since 2006 Green parties started to join governments opposed by the Social-Democrats (although the first experiments were punished by the voters at the following elections in Czechia, Finland and Ireland [Van Haute 2016]).
Data from ‘a’ to ‘d’ will be collected by European open databases (including Eurobarometer), literature on media, electoral data and national official sources. Data from ‘e’ to g’ will be provided by literature and Green parties’ official releases.
The hypothesis is that the external factors are necessary but not sufficient to fully explain the (unequal) rise of Green parties in Europe in the 21st Century. It will be tested in a comparative research, selecting eight couples of neighbouring countries:
• Finland, Sweden (Scandinavia);
• Ireland, United Kingdom (British Islands);
• France, Portugal (Atlantic),
• Belgium, Luxembourg (BENELUX);
• Austria, Germany (Centre);
• Italy, Spain (Mediterranean);
• Czechia, Poland (East);
• Bulgaria, Romania (Balkans).
Keeping the door open to the possible identification of other issues by data, the comparisons’ results will show which factors seem to be more relevant to explain the Greens’ growth and the current divergence between European ecologist parties, confirming or not the hypothesis of the decisive role played by the party’s own development and the regional differences.